sub mult_coeff {
   my $c = shift();

   return [] if $c <= 0;
   return [ [ @_ ] ] if $c == 1;

   my @sets;
   for my $k (1 .. @_ - $c + 1)
   {
       for my $nck_ref (nck_with_leftover($k, @_))
       {
           push @sets,
               map {
                   unshift(@{ $_ }, [ @{ $nck_ref->[0] } ]); # clone r
+eference
                   $_
               } mult_coeff($c - 1, @{ $nck_ref->[1] });
       }
   }

   return @sets;
}

sub nck_with_leftover {
   my $k = shift();

   return [ [], [ @_ ] ] if $k <= 0;

   my @groups;
   my @leftover;
   while (@_)
   {
       my $pick = shift();
       push @groups,
           map {
               unshift(@{ $_->[0] }, $pick);
               unshift(@{ $_->[1] }, @leftover);
               $_
           } nck_with_leftover($k - 1, @_);
       push @leftover, $pick;
   }

   return @groups;
}

use Data::Dumper;
my @results = mult_coeff(3, qw(a b c d e));
print Dumper \@results;
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