use strict;
use warnings;
use Math::Combinatorics;
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?';

print display($_, $firstLetter), "\n"
  for grep qualify($_, $subset),
    Math::Combinatorics::combine(4,split ",",$str);

sub qualify {
  my($combo, $subset) = @_;

  # Check if numbers are all unique, using the "postincrement" trick
  my %seen;
  $seen{$_}++ or return 0 for map substr($_, 2), @$combo;

  # Find all the tokens in the combo. If all the ones we need in the
  # subset are represented, we're good.
  my %token = map +(substr($_, 0, 2) => 1), @$combo;
  return all { $token{$_} } split /,/, $subset;
}

sub display {
  my($combo, $first) = @_;
  # Use a Schwartzian transform over a reverse sort on our
  # calculated "firstness weight": -1 for elements with the wrong
  # first letter, else the element's weight. sort() is overkill here,
  # I'd use a different approach if we were considering larger sets.
  return join ',',
    map $_->[0],
      sort { $b->[1] <=> $a->[1] }
        map [ $_, substr($_, 0, 1) eq $first ? substr($_, 2) : -1 ],
          @$combo;
}

##</code><code>##

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