my %forwardhash = (
    key1 => 'value1',
    key2 => 'value2',
    key3 => 'value3'
  );

  my %reversehash = ();
  while ( my ( $key, $value ) = each %forwardhash ) {
      $reversehash{$value} = $key;
  }

  use Data::Dumper;
  print Dumper( \%reversehash );