Use typeglobs. They're almost as good as caramel corn:

#!/usr/bin/perl -w

use strict;

sub here {
}

foreach my $poss ( qw( here not_here ) ) {
    no strict 'refs';
    if (*{$poss}{CODE}) {
        print "$poss exists.\n";
    } else {
        print "$poss does not.\n";
    }
}
[download]

What this does is look for a typeglob of a certain name. (That's the *{$poss} part.) Then it looks to see if the CODE slot is defined. I think you can take it from there.

Update: I just read the 'reference' part. If it's an anonymous reference, that's trouble. (I think you'd have to get access to Perl guts to find it. Yikes.) If it's not, you can check the symbol table for a match. There's probably a better way to do this with grep, but this is illustrative:

#!/usr/bin/perl -w

use strict;

sub findme {
    return 0;
}

sub another {}

my $ref;

if (rand(1) < 0.5) {
    $ref = \&findme;
} else {
    $ref = \&another;
}

foreach (keys %main::) {
    no strict 'refs';
    if (defined *{$_}{CODE}) {
        if (*{$_}{CODE} eq $ref) {
             print "\$ref points to $_!\n";
        }
    }
}
[download]

It's not beautiful, and you'll have to have some idea which package the sub might be in, but it's a little closer.

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