my %x = ( 1 => [qw( o o p s )],
          2 => [qw( j u s t )],
          3 => [qw( a n o t h e r )],
          4 => [qw( h a s h )],          
          5 => [qw( e r r o r )],
          6 => [qw( j u s t  a n o t h e r  p e r l  h a c k e r , )]
        );

my %rev = map { my $key=$_; map { $_=>$key} @{$x{$key}} } keys %x;
print "$_ -> $rev{$_}\n" foreach sort keys %rev;