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use strict;
use warnings;
...
        map [ $_, substr($_, 0, 1) eq $first ? substr($_, 2) : -1 ],
          @$combo;
}

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