my %hash = (
  a => 1,
  b => 1,
  c => 1,
  d => 2,
  e => 2,
  f => 2,
);

my %by_value;

while(my($key, $value) = each %hash) {
  push @{ $by_value{$value} }, $key;
}

print Dumper(\%by_value);