use strict;
use warnings;

sub process_token {
   my ($token) = @_;
   print("Token: $token\n");
}

{
   my $str = "ABC[GHI]XY[Z]1A";

   for ($str) {
      /\G \[ ([A-Z]*) \] /xgcs && do {
         process_token("$1");
         redo
      };

      /\G ([A-Z]) /xgcs && do {
         process_token("$1"); 
         redo
      };

      /\G (.) /xgcs && do {
         printf("Unexpected '%s' at pos %d\n", $1, pos()-length($1));
         redo
      };
   }
}