#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

sub My::Filters::known {}
sub My::Filters::forward;

for my $string (qw(My::Filters::known
                   My::Filters::unknown
                   My::Filters::forward)
) {
    say join "\t", $string, defined &$string, exists &$string;
}

##</code><code>##

My::Filters::known	1	1
My::Filters::unknown		
My::Filters::forward		1