use strict;

my %hash = (
    1 => 'a'
,   2 => 'b'
,   3 => 'c'
,   4 => 'b'
,   5 => 'e'
,   6 => 'b'
,   7 => 'a'
,   8 => 'z'
);

my %values_with_multiple_keys = ();

foreach my $k (keys %hash) {
    push @{ $values_with_multiple_keys{$hash{$k}} }, $k;
}

foreach my $m (keys %values_with_multiple_keys) {
    if (scalar @{$values_with_multiple_keys{$m}} > 1) {
        print "$m is referenced by: ", join ', ', @{$values_with_multiple_keys{$m}}, "\n";
    }
}
___OUTPUT___
a is referenced by: 7, 1, 
b is referenced by: 6, 2, 4,