constant - Perl pragma to declare constants


use constant PI    => 4 * atan2(1, 1);
use constant DEBUG => 0;

print "Pi equals ", PI, "...\n" if DEBUG;

use constant {
    SEC   => 0,
    MIN   => 1,
    HOUR  => 2,
    MDAY  => 3,
    MON   => 4,
    YEAR  => 5,
    WDAY  => 6,
    YDAY  => 7,
    ISDST => 8,

use constant WEEKDAYS => qw(
    Sunday Monday Tuesday Wednesday Thursday Friday Saturday

print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";


This pragma allows you to declare constants at compile-time.


When a constant is used in an expression, Perl replaces it with its value at compile time, and may then optimize the expression further. In particular, any code in an if (CONSTANT) block will be optimized away if the constant is false.



Constants defined using this module cannot be interpolated into strings like variables. However, concatenation works just fine:

print "Pi equals PI...\n";        # WRONG: does not expand "PI"
print "Pi equals ".PI."...\n";    # right

Even though a reference may be declared as a constant, the reference may point to data which may be changed, as this code shows.

use constant ARRAY => [ 1,2,3,4 ];
print ARRAY->[1];
ARRAY->[1] = " be changed";
print ARRAY->[1];

Dereferencing constant references incorrectly (such as using an array subscript on a constant hash reference, or vice versa) will be trapped at compile time.

Constants belong to the package they are defined in. To refer to a constant defined in another package, specify the full package name, as in Some::Package::CONSTANT. Constants may be exported by modules, and may also be called as either class or instance methods, that is, as Some::Package->CONSTANT or as $obj->CONSTANT where $obj is an instance of Some::Package. Subclasses may define their own constants to override those in their base class.

The use of all caps for constant names is merely a convention, although it is recommended in order to make constants stand out and to help avoid collisions with other barewords, keywords, and subroutine names. Constant names must begin with a letter or underscore. Names beginning with a double underscore are reserved. Some poor choices for names will generate warnings, if warnings are enabled at compile time.

List constants

Constants may be lists of more (or less) than one value. A constant with no values evaluates to undef in scalar context. Note that constants with more than one value do not return their last value in scalar context as one might expect. They currently return the number of values, but this may change in the future. Do not use constants with multiple values in scalar context.

NOTE: This implies that the expression defining the value of a constant is evaluated in list context. This may produce surprises:

use constant TIMESTAMP => localtime;                # WRONG!
use constant TIMESTAMP => scalar localtime;         # right

The first line above defines TIMESTAMP as a 9-element list, as returned by localtime() in list context. To set it to the string returned by localtime() in scalar context, an explicit scalar keyword is required.

List constants are lists, not arrays. To index or slice them, they must be placed in parentheses.

my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
my @workdays = (WEEKDAYS)[1 .. 5];          # right

Defining multiple constants at once

Instead of writing multiple use constant statements, you may define multiple constants in a single statement by giving, instead of the constant name, a reference to a hash where the keys are the names of the constants to be defined. Obviously, all constants defined using this method must have a single value.

use constant {
    FOO => "A single value",
    BAR => "This", "won't", "work!",        # Error!

This is a fundamental limitation of the way hashes are constructed in Perl. The error messages produced when this happens will often be quite cryptic -- in the worst case there may be none at all, and you'll only later find that something is broken.

When defining multiple constants, you cannot use the values of other constants defined in the same declaration. This is because the calling package doesn't know about any constant within that group until after the use statement is finished.

use constant {
    NEGMASK => ~BITMASK,                    # Error!

Magic constants


use constant E2BIG => ($! = 7);

You can't produce a tied constant by giving a tied scalar as the value. References to tied variables, however, can be used as constants without any problems.



In the rare case in which you need to discover at run time whether a particular constant has been declared via this module, you may use this function to examine the hash %constant::declared. If the given constant name does not include a package name, the current package is used.

sub declared ($) {
    use constant 1.01;              # don't omit this!
    my $name = shift;
    $name =~ s/^::/main::/;
    my $pkg = caller;
    my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";


List constants are not inlined unless you are using Perl v5.20 or higher. In v5.20 or higher, they are still not read-only, but that may change in future versions.

It is not possible to have a subroutine or a keyword with the same name as a constant in the same package. This is probably a Good Thing.

A constant with a name in the list STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG is not allowed anywhere but in package main::, for technical reasons.


You can get into trouble if you use constants in a context which automatically quotes barewords (as is true for any subroutine call). For example, you can't say $hash{CONSTANT} because CONSTANT will be interpreted as a string. Use $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in. Similarly, since the => operator quotes a bareword immediately to its left, you have to say CONSTANT() => 'value' (or simply use a comma in place of the big arrow) instead of CONSTANT => 'value'.


Readonly - Facility for creating read-only scalars, arrays, hashes.

Attribute::Constant - Make read-only variables via attribute

Scalar::Readonly - Perl extension to the SvREADONLY scalar flag

Hash::Util - A selection of general-utility hash subroutines (mostly to lock/unlock keys and values)


Please report any bugs or feature requests via the perlbug(1) utility.



Multiple constant declarations at once added by Casey West, <>.

Documentation mostly rewritten by Ilmari Karonen, <>.

This program is maintained by the Perl 5 Porters. The CPAN distribution is maintained by Sébastien Aperghis-Tramoni <>.


Copyright (C) 1997, 1999 Tom Phoenix
