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# always use stric and warnings (till the moment you know when is safe
+ disabling them)
use strict;
...
X_202
X_203
X_204

    my $cur_num = $1 if $arr[$_] =~/\d*[A-Z]_(\d+)$/;
    my $next_num = $1 if $arr[$_ + 1] =~/\d*[A-Z]_(\d+)$/;

    my ($cur_num,$next_num) = map {$1 if $_  =~/\d*[A-Z]_(\d+)$/} $arr
+[$_],$arr[$_+1];

   if (  $arr[$_] =~/\d*[A-Z]_(\d+)$/  ){
       $cur_num = $1;
   }