DB<140> \%h
 => { a => 1, b => 2, c => 1, d => 2, e => 3 }

  DB<141> push @{ $h2{$h{$_}} }, $_ for keys %h
 => ""

  DB<142> \%h2
 => { 1 => ["c", "a"], 2 => ["b", "d"], 3 => ["e"] }