##
@unique_keys = do { my $hash = reverse %myhash; values %hash };
##
##
%myhash = (1 => 'a', 2=> 'b', 3=> 'a', 4=>'c',5 => 'b', 6 => 'a');
@k = do {
my %s;
map {
my $v=$_;
grep { $myhash{$_} eq $v} keys %myhash
} grep {$s{$_}++==1} values %myhash
};
print "$_ => $myhash{$_}\n" for @k;
__END__
6 => a
1 => a
3 => a
2 => b
5 => b