my %hash = (1 => 'One', 2 => 'Two', 3 => 'One'); # fixed: not {}
my %seen;

foreach my $key (keys %hash) {
	push @{$seen{$hash{$key}}}, $key;
}

foreach (keys %seen) {
	print "Keys having $_ as value: " . join(', ', @{$seen{$_}}) . "\n" if @{$seen{$_}} > 1;
}