gistfile1.pl

use strict;
use warnings;
use Math::Combinatorics qw(combine);
use List::Util qw(sum);

my $target = shift || 0;

my %pairs = (
  #a => 1, b => 2, c => 3,
  #d => 2, e => 1, f => 0,
  #g => 4, h => 8, i => 7,
  #j => 10
  ( a => 1, b => 7, c => 3, d => 5 )
);

my %sums;
my $k = 1;
while($k <= keys %pairs){
  my @sets = combine($k, keys %pairs);
  foreach my $s(@sets){
    my $key = join(' + ', sort @$s);
    if(sum(@pairs{@$s}) == $target){
      $sums{$key} = $target;
    }
  }
  $k++;
}

foreach my $key(sort {$a cmp $b || length($a) <=> length($b) } keys %sums) {
  print "$key == $sums{$key}\n" if $sums{$key} == $target || 1;
}