#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11145154
use warnings;
use ntheory qw( forcomb );
use List::Util qw( all );

my $firstLetter = 'c';
my $str = 'a?1,a?2,a%3,b?2,b?3,b%5,c%4,c%5,c?6,d%2';
my $subset = 'a?,b%,c?';

my @str = sort { $b =~ $firstLetter <=> $a =~ $firstLetter or # sort only once
  $b =~ tr/0-9//cdr <=> $a =~ tr/0-9//cdr } split /,/, $str;
forcomb
  {
  my $comb = join ',', @str[@_];
  $comb =~ /\b(\d+)\b.*\b\1\b/ or                  # duplicate number
    all { $comb =~ /\Q$_/ } split /,/, $subset and # each subset
    print "$comb\n";
  } scalar @str, 4;

##</code><code>##

c?6,c%4,b%5,a?2
c?6,c%4,b%5,a?1
c?6,b%5,a%3,a?2
c?6,b%5,a%3,a?1
c?6,b%5,b?3,a?2
c?6,b%5,b?3,a?1
c?6,b%5,a?2,a?1
c?6,b%5,b?2,a?1
c?6,b%5,d%2,a?1