just found an "if pragma" which could be used for backwards compatibility!

Ah, but if.pm has backwards compatability issues itself - it first appeared in 5.6.2^† Here's what I use if I want to use warnings, but still want the code to be able to run in 5.005 (without warnings enabled of course):

BEGIN {
    $INC{"warnings.pm"} = 1 if $] < 5.006;
}
use strict;
use warnings;
[download]

Note that if you want to be able to do no warnings, you have to create a dummy sub warnings::unimport as well.

^†I've no reason to assume if.pm cannot be installed in 5.005. I just don't want another module dependency for what I can do in a single statement myself.