Next attempt: 

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;

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In reply to Re^3: inverting hash / grouping values by hdb
in thread inverting hash / grouping values by LanX

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