perlfaq7 - General Perl Language Issues ($Revision: 1.6 $,
$Date: 2003/12/03 03:02:45 $)
This section deals with general Perl language issues that
don't clearly fit into any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc
grammar in perly.y in the source distribution if you're
particularly brave. The grammar relies on very smart tokenizing
code, so be prepared to venture into toke.c as
well.
In the words of Chaim Frenkel: "Perl's grammar can not be
reduced to BNF. The work of parsing perl is distributed
between yacc, the lexer, smoke and mirrors."
What are all these $@%&* punctuation signs, and how do I
know when to use them?
They are type specifiers, as detailed in perldata:
$ for scalar values (number, string or reference)
@ for arrays
% for hashes (associative arrays)
& for subroutines (aka functions, procedures, methods)
* for all types of that symbol name. In version 4 you
used them like
pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to
encounter that aren't really type specifiers:
<> are used for inputting a record from a filehandle.
takes a reference to something.
Note that <FILE> is neither the type specifier for files
nor the name of the handle. It is the "<>" operator
applied to the handle FILE. It reads one line (well,
record--see "$/" in perlvar) from the handle FILE in
scalar context, or all lines in list context. When performing
open, close, or any other operation besides "<>"
on files, or even when talking about the handle, do not
use the brackets. These are correct: "eof(FH)", "seek(FH,
0, 2)" and "copying from STDIN to FILE".
Do I always/never have to quote my strings or use semi-
colons and commas?
Normally, a bareword doesn't need to be quoted, but in
most cases probably should be (and must be under "use
strict"). But a hash key consisting of a simple word
(that isn't the name of a defined subroutine) and the
left-hand operand to the "=>" operator both count as
though they were quoted:
This is like this
------------ ---------------
$foo{line} $foo{"line"}
bar => stuff "bar" => stuff
The final semicolon in a block is optional, as is the
final comma in a list. Good style (see perlstyle) says to
put them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
How do I skip some return values?
One way is to treat the return values as a list and index
into it:
$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the
left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements
that you need:
($dev, $ino, $uid, $gid) = ( stat($file)
)[0,1,4,5];
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the "use warnings"
pragma allows fine control of what warning are produced.
See perllexwarn for more details.
{
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
If you have an older version of Perl, the $^W variable
(documented in perlvar) controls runtime warnings for a
block:
{
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Note that like all the punctuation variables, you cannot
currently use my() on $^W, only local().
What's an extension?
An extension is a way of calling compiled C code from
Perl. Reading perlxstut is a good place to learn more
about extensions.
Why do Perl operators have different precedence than C [Toc] [Back]
operators?
Actually, they don't. All C operators that Perl copies
have the same precedence in Perl as they do in C. The
problem is with operators that C doesn't have, especially
functions that give a list context to everything on their
right, eg. print, chmod, exec, and so on. Such functions
are called "list operators" and appear as such in the
precedence table in perlop.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or
use the super low precedence "or" operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "not")
deliberately have precedence lower than that of list operators
for just such situations as the one above.
Another operator with surprising precedence is exponentiation.
It binds more tightly even than unary minus, making
"-2**2" product a negative not a positive four. It is
also right-associating, meaning that "2**3**2" is two
raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:"
operator produces an lvalue. This assigns $x to either $a
or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a
(probably anonymous) hash reference. See perlref and
perldsc for details. Here's an example:
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to
"Nat"
If you're looking for something a bit more rigorous, try
perltoot.
How do I create a module?
A module is a package that lives in a file of the same
name. For example, the Hello::There module would live in
Hello/There.pm. For details, read perlmod. You'll also
find Exporter helpful. If you're writing a C or mixedlanguage
module with both C and Perl, then you should
study perlxstut.
The "h2xs" program will create stubs for all the important
stuff for you:
% h2xs -XA -n My::Module
The "-X" switch tells "h2xs" that you are not using "XS"
extension code. The "-A" switch tells "h2xs" that you are
not using the AutoLoader, and the "-n" switch specifies
the name of the module. See h2xs for more details.
How do I create a class?
See perltoot for an introduction to classes and objects,
as well as perlobj and perlbot.
How can I tell if a variable is tainted?
You can use the tainted() function of the Scalar::Util
module, available from CPAN (or included with Perl since
release 5.8.0). See also "Laundering and Detecting
Tainted Data" in perlsec.
What's a closure?
Closures are documented in perlref.
Closure is a computer science term with a precise but
hard-to-explain meaning. Closures are implemented in Perl
as anonymous subroutines with lasting references to lexical
variables outside their own scopes. These lexicals
magically refer to the variables that were around when the
subroutine was defined (deep binding).
Closures make sense in any programming language where you
can have the return value of a function be itself a function,
as you can in Perl. Note that some languages provide
anonymous functions but are not capable of providing
proper closures: the Python language, for example. For
more information on closures, check out any textbook on
functional programming. Scheme is a language that not
only supports but encourages closures.
Here's a classic function-generating function:
sub add_function_generator {
return sub { shift + shift };
}
$add_sub = add_function_generator();
$sum = $add_sub->(4,5); # $sum is 9
now.
The closure works as a function template with some customization
slots left out to be filled later. The anonymous
subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals
outside its own scope.
Contrast this with the following make_adder() function, in
which the returned anonymous function contains a reference
to a lexical variable outside the scope of that function
itself. Such a reference requires that Perl return a
proper closure, thus locking in for all time the value
that the lexical had when the function was created.
sub make_adder {
my $addpiece = shift;
return sub { shift + $addpiece };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in,
whereas "&$f2($n)" is always 555 plus whatever $n you pass
in. The $addpiece in the closure sticks around.
Closures are often used for less esoteric purposes. For
example, when you want to pass in a bit of code into a
function:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
'$line = <STDIN>', there would have been no way for the
hypothetical timeout() function to access the lexical
variable $line back in its caller's scope.
What is variable suicide and how can I prevent it?
Variable suicide is when you (temporarily or permanently)
lose the value of a variable. It is caused by scoping
through my() and local() interacting with either closures
or aliased foreach() iterator variables and subroutine
arguments. It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder. Take
this code:
my $f = "foo";
sub T {
while ($i++ < 3) { my $f = $f; $f .= "bar"; print
$f, "0 }
}
T;
print "Finally $f0;
The $f that has "bar" added to it three times should be a
new $f ("my $f" should create a new local variable each
time through the loop). It isn't, however. This was a
bug, now fixed in the latest releases (tested against
5.004_05, 5.005_03, and 5.005_56).
How can I pass/return a {Function, FileHandle, Array,
Hash, Method, Regex}?
With the exception of regexes, you need to pass references
to these objects. See "Pass by Reference" in perlsub for
this particular question, and perlref for information on
references.
See ``Passing Regexes'', below, for information on passing
regular expressions.
Passing Variables and Functions
Regular variables and functions are quite easy to
pass: just pass in a reference to an existing or
anonymous variable or function:
func( ome_scalar );
func( @some_array );
func( [ 1 .. 10 ] );
func( some_hash );
func( { this => 10, that => 20 } );
func( some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
As of Perl 5.6, you can represent filehandles with
scalar variables which you treat as any other scalar.
open my $fh, $filename or die "Cannot open
$filename! $!";
func( $fh );
sub func {
my $passed_fh = shift;
my $line = <$fh>;
}
Before Perl 5.6, you had to use the *FH or "H"
notations. These are "typeglobs"--see "Typeglobs and
Filehandles" in perldata and especially "Pass by Reference"
in perlsub for more information.
Passing Regexes
To pass regexes around, you'll need to be using a
release of Perl sufficiently recent as to support the
"qr//" construct, pass around strings and use an
exception-trapping eval, or else be very, very clever.
Here's an example of how to pass in a string to be
regex compared using "qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
Notice how "qr//" allows flags at the end. That pattern
was compiled at compile time, although it was
executed later. The nifty "qr//" notation wasn't
introduced until the 5.005 release. Before that, you
had to approach this problem much less intuitively.
For example, here it is again if you don't have
"qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
}
$match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
return eval "al =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due
to the double interpolation of the eval and the double-quoted
string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &")
} danger';
eval "tring =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the
O'Reilly book, Mastering Regular Expressions, by Jeffrey
Friedl. Page 273's Build_MatchMany_Function() is
particularly interesting. A complete citation of this
book is given in perlfaq2.
Passing Methods
To pass an object method into a subroutine, you can do
this:
call_a_lot(10, $some_obj, "methname")
sub call_a_lot {
my ($count, $widget, $trick) = @_;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
}
}
Or, you can use a closure to bundle up the object, its
method call, and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args)
};
func($whatnot);
sub func {
my $code = shift;
&$code();
}
You could also investigate the can() method in the
UNIVERSAL class (part of the standard perl distribution).
How do I create a static variable?
As with most things in Perl, TMTOWTDI. What is a "static
variable" in other languages could be either a functionprivate
variable (visible only within a single function,
retaining its value between calls to that function), or a
file-private variable (visible only to functions within
the file it was declared in) in Perl.
Here's code to implement a function-private variable:
BEGIN {
my $counter = 42;
sub prev_counter { return --$counter }
sub next_counter { return $counter++ }
}
Now prev_counter() and next_counter() share a private
variable $counter that was initialized at compile time.
To declare a file-private variable, you'll still use a
my(), putting the declaration at the outer scope level at
the top of the file. Assume this is in file Pax.pm:
package Pax;
my $started = scalar(localtime(time()));
sub begun { return $started }
When "use Pax" or "require Pax" loads this module, the
variable will be initialized. It won't get garbage-collected
the way most variables going out of scope do,
because the begun() function cares about it, but no one
else can get it. It is not called $Pax::started because
its scope is unrelated to the package. It's scoped to the
file. You could conceivably have several packages in that
same file all accessing the same private variable, but
another file with the same package couldn't get to it.
See "Persistent Private Variables" in perlsub for details.
What's the difference between dynamic and lexical (static)
scoping? Between local() and my()?
"local($x)" saves away the old value of the global variable
$x and assigns a new value for the duration of the
subroutine which is visible in other functions called from
that subroutine. This is done at run-time, so is called
dynamic scoping. local() always affects global variables,
also called package variables or dynamic variables.
"my($x)" creates a new variable that is only visible in
the current subroutine. This is done at compile-time, so
it is called lexical or static scoping. my() always
affects private variables, also called lexical variables
or (improperly) static(ly scoped) variables.
For instance:
sub visible {
print "var has value $var0;
}
sub dynamic {
local $var = 'local'; # new temporary value for
the still-global
visible(); # variable called $var
}
sub lexical {
my $var = 'private'; # new private variable,
$var
visible(); # (invisible outside of
sub scope)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get
printed. That's because $var only has that value within
the block of the lexical() function, and it is hidden from
called subroutine.
In summary, local() doesn't make what you think of as private,
local variables. It gives a global variable a temporary
value. my() is what you're looking for if you want
private variables.
See "Private Variables via my()" in perlsub and "Temporary
Values via local()" in perlsub for excruciating details.
How can I access a dynamic variable while a similarly [Toc] [Back]
named lexical is in scope?
If you know your package, you can just mention it explicitly,
as in $Some_Pack::var. Note that the notation $::var
is not the dynamic $var in the current package, but rather
the one in the "main" package, as though you had written
$main::var.
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var0;
print "global is $main::var0;
Alternatively you can use the compiler directive our() to
bring a dynamic variable into the current lexical scope.
require 5.006; # our() did not exist before 5.6
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var0;
{
our $var;
print "global is $var0;
}
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous
subroutines are the same ones that were in scope when the
subroutine was created. In shallow binding, they are
whichever variables with the same names happen to be in
scope when the subroutine is called. Perl always uses
deep binding of lexical variables (i.e., those created
with my()). However, dynamic variables (aka global,
local, or package variables) are effectively shallowly
bound. Consider this just one more reason not to use
them. See the answer to "What's a closure?".
Why doesn't "my($foo) = <FILE>;" work right?
"my()" and "local()" give list context to the right hand
side of "=". The <FH> read operation, like so many of
Perl's functions and operators, can tell which context it
was called in and behaves appropriately. In general, the
scalar() function can help. This function does nothing to
the data itself (contrary to popular myth) but rather
tells its argument to behave in whatever its scalar fashion
is. If that function doesn't have a defined scalar
behavior, this of course doesn't help you (such as with
sort()).
To enforce scalar context in this particular case, however,
you need merely omit the parentheses:
local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway,
although the issue is the same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
How do I redefine a builtin function, operator, or method?
Why do you want to do that? :-)
If you want to override a predefined function, such as
open(), then you'll have to import the new definition from
a different module. See "Overriding Built-in Functions"
in perlsub. There's also an example in "Class::Template"
in perltoot.
If you want to overload a Perl operator, such as "+" or
"**", then you'll want to use the "use overload" pragma,
documented in overload.
If you're talking about obscuring method calls in parent
classes, see "Overridden Methods" in perltoot.
What's the difference between calling a function as &foo
and foo()?
When you call a function as &foo, you allow that function
access to your current @_ values, and you bypass prototypes.
The function doesn't get an empty @_--it gets
yours! While not strictly speaking a bug (it's documented
that way in perlsub), it would be hard to consider this a
feature in most cases.
When you call your function as "&foo()", then you do get a
new @_, but prototyping is still circumvented.
Normally, you want to call a function using "foo()". You
may only omit the parentheses if the function is already
known to the compiler because it already saw the definition
("use" but not "require"), or via a forward reference
or "use subs" declaration. Even in this case, you get a
clean @_ without any of the old values leaking through
where they don't belong.
How do I create a switch or case statement?
This is explained in more depth in the perlsyn. Briefly,
there's no official case statement, because of the variety
of tests possible in Perl (numeric comparison, string comparison,
glob comparison, regex matching, overloaded comparisons,
...). Larry couldn't decide how best to do
this, so he left it out, even though it's been on the wish
list since perl1.
Starting from Perl 5.8 to get switch and case one can use
the Switch extension and say:
use Switch;
after which one has switch and case. It is not as fast as
it could be because it's not really part of the language
(it's done using source filters) but it is available, and
it's very flexible.
But if one wants to use pure Perl, the general answer is
to write a construct like this:
for ($variable_to_test) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
}
Here's a simple example of a switch based on pattern
matching, this time lined up in a way to make it look more
like a switch statement. We'll do a multiway conditional
based on the type of reference stored in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
/CODE/ && do {
warn "can't print function
ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
See "perlsyn/"Basic BLOCKs and Switch Statements"" for
many other examples in this style.
Sometimes you should change the positions of the constant
and the variable. For example, let's say you wanted to
test which of many answers you were given, but in a caseinsensitive
way that also allows abbreviations. You can
use the following technique if the strings all start with
different characters or if you want to arrange the matches
so that one takes precedence over another, as "SEND" has
precedence over "STOP" here:
chomp($answer = <>);
if ("SEND" =~ /^Q$answer/i) { print "Action is
send0 }
elsif ("STOP" =~ /^Q$answer/i) { print "Action is
stop0 }
elsif ("ABORT" =~ /^Q$answer/i) { print "Action is
abort0 }
elsif ("LIST" =~ /^Q$answer/i) { print "Action is
list0 }
elsif ("EDIT" =~ /^Q$answer/i) { print "Action is edit0 }
A totally different approach is to create a hash of function
references.
my %commands = (
"happy" => joy,
"sad", => sullen,
"done" => sub { die "See ya!" },
"mad" => angry,
);
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string0;
}
How can I catch accesses to undefined variables, func-
tions, or methods?
The AUTOLOAD method, discussed in "Autoloading" in perlsub
and "AUTOLOAD: Proxy Methods" in perltoot, lets you capture
calls to undefined functions and methods.
When it comes to undefined variables that would trigger a
warning under "use warnings", you can promote the warning
to an error.
use warnings FATAL => qw(uninitialized);
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused,
you've misspelled the method name, or the object is
of the wrong type. Check out perltoot for details about
any of the above cases. You may also use "print
ref($object)" to find out the class $object was blessed
into.
Another possible reason for problems is because you've
used the indirect object syntax (eg, "find Guru "Samy"")
on a class name before Perl has seen that such a package
exists. It's wisest to make sure your packages are all
defined before you start using them, which will be taken
care of if you use the "use" statement instead of
"require". If not, make sure to use arrow notation (eg.,
"Guru->find("Samy")") instead. Object notation is
explained in perlobj.
Make sure to read about creating modules in perlmod and
the perils of indirect objects in "Method Invocation" in
perlobj.
How can I find out my current package?
If you're just a random program, you can do this to find
out what the currently compiled package is:
my $packname = __PACKAGE__;
But, if you're a method and you want to print an error
message that includes the kind of object you were called
on (which is not necessarily the same as the one in which
you were compiled):
sub amethod {
my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
How can I comment out a large block of perl code?
You can use embedded POD to discard it. Enclose the
blocks you want to comment out in POD markers, for example
"=for nobody" and "=cut" (which marks ends of POD blocks).
# program is here
=for nobody
all of this stuff
here will be ignored
by everyone
=cut
# program continues
The pod directives cannot go just anywhere. You must put
a pod directive where the parser is expecting a new statement,
not just in the middle of an expression or some
other arbitrary grammar production.
See perlpod for more details.
How do I clear a package?
Use this code, provided by Mark-Jason Dominus:
sub scrub_package {
no strict 'refs';
my $pack = shift;
die "Shouldn't delete main package"
if $pack eq "" || $pack eq "main";
my $stash = *{$pack . '::'}{HASH};
my $name;
foreach $name (keys %$stash) {
my $fullname = $pack . '::' . $name;
# Get rid of everything with that name.
undef $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
}
}
Or, if you're using a recent release of Perl, you can just
use the Symbol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain
the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two
reasons.
The first reason is that this technique only works on
global variables. That means that if $fred is a lexical
variable created with my() in the above example, the code
wouldn't work at all: you'd accidentally access the global
and skip right over the private lexical altogether.
Global variables are bad because they can easily collide
accidentally and in general make for non-scalable and confusing
code.
Symbolic references are forbidden under the "use strict"
pragma. They are not true references and consequently are
not reference counted or garbage collected.
The other reason why using a variable to hold the name of
another variable is a bad idea is that the question often
stems from a lack of understanding of Perl data structures,
particularly hashes. By using symbolic references,
you are just using the package's symbol-table hash (like
%main::) instead of a user-defined hash. The solution is
to use your own hash or a real reference instead.
$USER_VARS{"fred"} = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic
references. Sometimes this comes up in reading strings
from the user with variable references and wanting to
expand them to the values of your perl program's variables.
This is also a bad idea because it conflates the
program-addressable namespace and the user-addressable
one. Instead of reading a string and expanding it to the
actual contents of your program's own variables:
$str = 'this has a $fred and $barney in it';
$str =~ s/(w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS
and have variable references actually refer to entries in
that hash:
$str =~ s/+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous
approach. Of course, you don't need to use a dollar sign.
You could use your own scheme to make it less confusing,
like bracketed percent symbols, etc.
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(1008
Another reason that folks sometimes think they want a
variable to contain the name of a variable is because they
don't know how to build proper data structures using
hashes. For example, let's say they wanted two hashes in
their program: %fred and %barney, and that they wanted to
use another scalar variable to refer to those by name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled
with the problems enumerated above. It would be far better
to write:
$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic references
are when you really must refer to the symbol
table. This may be because it's something that can't take
a real reference to, such as a format name. Doing so may
also be important for method calls, since these always go
through the symbol table for resolution.
In those cases, you would turn off "strict 'refs'" temporarily
so you can play around with the symbol table.
For example:
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
no strict 'refs'; # renege for the block
*$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
All those functions (red(), blue(), green(), etc.) appear
to be separate, but the real code in the closure actually
was compiled only once.
So, sometimes you might want to use symbolic references to
directly manipulate the symbol table. This doesn't matter
for formats, handles, and subroutines, because they are
always global--you can't use my() on them. For scalars,
arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
What does "bad interpreter" mean?
The "bad interpreter" message comes from the shell, not
perl. The actual message may vary depending on your platform,
shell, and locale settings.
If you see "bad interpreter - no such file or directory",
the first line in your perl script (the "shebang" line)
does not contain the right path to perl (or any other program
capable of running scripts). Sometimes this happens
when you move the script from one machine to another and
each machine has a different path to perl---/usr/bin/perl
versus /usr/local/bin/perl for instance.
If you see "bad interpreter: Permission denied", you need
to make your script executable.
In either case, you should still be able to run the
scripts with perl explicitly:
% perl script.pl
If you get a message like "perl: command not found", perl
is not in your PATH, which might also mean that the
location of perl is not where you expect it so you need to
adjust your shebang line.
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in
this file are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
perl v5.8.5 2002-11-06 19 [ Back ] |
