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';
}
[download]

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 :(