Anonymous Monk,
If I understand your requirements:

• Take action if a inner key exists as an inner key in another hash.
• Know which value came from which hash/key
• Take default action if no match

The clearest way I can think to do this is to walk through the all the keys of the second hash for each inner key in the first hash. The only optimization I have done is return once a match is found.

#!/usr/bin/perl
use strict;
use warnings;

my %hash1 = (
    foo => { 'a' => 1, 'b' => 2, 'c' => 3 },
    bar => { 'd' => 1, 'e' => 2, 'f' => 3 },
);

my %hash2 = (
    blah => { 'm' => 1, 'd' => 2, 'a' => 3 },
    asdf => { 'z' => 1, 'l' => 2, 'b' => 3 },
);

for my $outer_key ( keys %hash1 ) {
    for my $inner_key ( keys %{ $hash1{$outer_key} } ) {
        my ($h2_outer, $h2_inner) = Find_Key( $inner_key , \%hash2 );
        if ( $h2_inner ) {
            # do something : $outer_key, $inner_key, $h2_outer, and $h
+2_inner
        }
        else {
            # do your other thing
        }
    }
}

sub Find_Key {
    my ($match, $hash) = @_;
    for my $key ( keys %$hash ) {
        for ( keys %{ $hash->{$key} } ) {
            return ($key, $_) if $_ eq $match;
        }
    }
    return (undef, undef);
}
[download]

If this code is not any clearer, please let me know and I will give a blow by blow explanation.

Cheers - L~R