DB<122> sub invert {
             my ($h)=@_;
             my %h2;
             while ( my ($k,$v) = each %$h ) { 
               push @{ $h2{$v} } , $k;
             } 
             return %h2;
          }

  DB<123> @t{a..f}=(1,2)x3
 => (1, 2, 1, 2, 1, 2)

  DB<124> \%t
 => { a => 1, b => 2, c => 1, d => 2, e => 1, f => 2 }

  DB<125> invert \%t
 => (1, ["e", "c", "a"], 2, ["b", "d", "f"])