Select Code to Download

   $input = '(E,(A,B),(C,(X,Y,Z),D),F)';
  $parsed = ['E,',['A,B'],',',['C,',['X,Y,Z'],',D'],',F'];
$filtered = ['E,',',',',F'];
  $output = '(E,F)';

use warnings;
use strict;
...
my $output = buildparens($filtered);
$output=~s/,(?=,)//g;
say "  \$output = ", Dumper($output), ";";