Depends. Readonly is slow, and not everyone can use Readonly::XS. The following code takes 48 seconds:

use Readonly;
Readonly $C1 => 1;

my $t0 = time();
my $a = 2;
for (1 .. 10000000) {
    if ($a ++ == $C1) {
        ;
    }
}
print time() - $t0;
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When this takes 4 second:

use constant C1 => 1;

my $t0 = time();
my $a = 2;
for (1 .. 10000000) {
    if ($a ++ == C1) {
        ;
    }
}
print time() - $t0;
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If performance is important, use constant.

I'd be happier developing with a function that called Readonly in development and then something like this in production:

sub fake_readonly {
  $_[0] = $_[1];
}
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And now you get most of the advantages of Readonly, but performance is not a problem. (I don't like constant because I'm a big fan of interpolating, and I often have to check whether someone has a constant or function because I'm afraid that it will try to suck up arguments.)

That's a difference of 4.4 microseconds. That's a difference I'm willing to take in most of my programs.

TheDamian's <cite>Best Practices</cite> gives several good reasons why constant should be deprecated in favour of Readonly:

Readonly my $LONG_COUNT => 14;
Readonly my $SHORT_COUNT => 9;
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But Damian also says in his book that you should use hashes for look-up tables, rather than if/else tests. And that's definitely a look-up table.

So how about something like:

Readonly \my %THWAPP_COUNT => (
  x => 14,
  y => 9,
);
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(Adjective describing what sort of a count this is courtesy of Acme::MetaSyntactic::batman.)

Smylers