What follows is a recursive solution. The expressions are paired, then the pairs are paired, then the paired pairs are paired, etc. until only one expression is left.

use strict;
use warnings;

my $command = '@MAC(output,X,Y,8);';


my ($fields) = $command =~ /\@MAC\((.*?)\)/;

my ($output, $var1, $var2, $size) =
   split(/\s*,\s*/, $fields);

my @data = map { "$var1$_*$var2$_"} (0 .. $size-1);

while (@data > 1) {
   my @compressed;

   push @compressed, sprintf("(%s)+(%s)", shift(@data), shift(@data))
      while @data;

   @data = @compressed;
}


print("$output = $data[0];\n");
[download]