use strict;
use warnings;

package ddd;

require Exporter;
our @ISA    = qw( Exporter );
our @EXPORT = qw( APPLE PEAR );

use constant APPLE => "funny";
use constant PEAR  => "bad";

1;
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use strict;
use warnings;

use ddd;

print "APPLE = " . APPLE . "\n";
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I don't use constant because of p. 55 in Perl Best Practices, authored by TheDamian. It recommends not using use constant at all, but the Readonly module instead. Since I can also install Readonly:XS, I don't have to worry about this module being slow with my scalars.

Same here. At work, we've recently removed "use constant" from our code and converted to Readonly due to Damian's advice. It's really cleaned up a lot of our code.

Unfortunately, there are some bugs in how Perl handle tie which Readonly exposes. For example, the following prints "no":

#!/usr/bin/perl -l

use strict;
use warnings;

use Readonly;
Readonly my $VAR => 'SEARCH';

foreach my $var ($VAR) {
    print +($var eq $VAR) ? 'yes' : 'no';
}
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Clearly that's wrong, but it's related to Perl's tie mechanism and not Readonly. As a workaround:

 print +($var.'' eq $VAR.'') ? 'yes' : 'no';

That's ugly as sin, but it works :(

Cheers,
Ovid

New address of my CGI Course.

Constants are different than normal variables, so I like how use constant constants look differently (due to lack of a sigil). Isn't that one of Perl's mantras?

Readonly uses tie. Tied variables are slower, and they behave erratically in a few situations.

On the other hand, Readonly works on types other than scalar.