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use strict;
use warnings;
...
      }
   }
}

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

my @strings = qw( a?1 a?2 a%3 b?2 b?3 b%5 c%4 c%5 c?6 d%2 a?7 c?7 );
my @mandatory_types = qw( a? b% c? );                   # ^^^^^^^
my $n = 5;
#       ^

a?7,c?6,b%5,a%3,a?1
a?7,c?6,b%5,b?3,a?1
...
a?7,c?6,b%5,c%4,a?1
a?7,c?6,b%5,a%3,a?1
a?7,c?6,b%5,b?3,a?1