use strict; use warnings; use List::Util 'reduce'; use Data::Dumper; my %t = ( a => 1, b => 2, c => 1, d => 2, e => 1, f => 2, g => 3 ); my $i = reduce { $a = { $t{$a} => [ $a ] } unless ref $a; push @{ $a->{ $t{$b} } }, $b; $a } keys %t; print Dumper $i;