use strict;
use warnings;
use Data::Dumper;

my ($k1, $k2, %foo, %bar, %fval, %bval);

%foo = (
         'a' => {
                  'x' => 1,
                  'y' => 3
                },
         'b' => {
                  'x' => 4,
                  'y' => 4
                }
       );
%bar = (
         'a' => {
                  'x' => 1,
                  'y' => 2
                },
         'b' => {
                  'x' => 3,
                  'y' => 4
                }
       );
for $k1 (keys %foo) {
    for $k2 (keys %{$foo{$k1}}) {
        push @{$fval{$foo{$k1}{$k2}}}, "$k1\t$k2";
    }
}
for $k1 (keys %bar) {
    for $k2 (keys %{$bar{$k1}}) {
        push @{$bval{$bar{$k1}{$k2}}}, "$k1\t$k2";
    }
}
for $k1 (sort keys %fval) {
    if (exists $bval{$k1}) {
        print "Value $k1 matches for:\n";
        for (sort @{$fval{$k1}}) {
             @_ = split /\t/;
             print "    \$foo{$_[0]}{$_[1]}\n";
        }
        for (sort @{$bval{$k1}}) {
             @_ = split /\t/;
             print "    \$bar{$_[0]}{$_[1]}\n";
        }
        print "\n";
    }
}