use strict;
use warnings;
use List::Util 'reduce';
use List::MoreUtils 'mesh';
use Data::Dumper;

my %t = ( a => 1, b => 2, c => 1, d => 2, e => 1, f => 2, g => 3 );

my( $k, $v ) = @{
	 +reduce { 
	   if( $a->[0]->[-1] eq $b->[0]->[0] ) {
	     push @{$a->[1]->[-1]}, $b->[1]->[0]->[0];
	   } else {
	     push @{$a->[0]}, $b->[0]->[0];
	     push @{$a->[1]}, [$b->[1]->[0]->[0]];
	   }
	   $a;
	 } 
	 map { [ [$t{$_}], [[$_]] ] } sort { $t{$a} <=> $t{$b} } keys %t };

my %i = mesh @$k, @$v;
print Dumper \%i;