if (grep { -f $_ and $_ eq $FilePath } @DirsArray){}
else{
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Is usually written as:

unless ( grep { -f $_ and $_ eq $FilePath } @DirsArray ) {
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   printf("\n" . $FilePath . "\n");
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Is better written as:

   print "\n" . $FilePath . "\n";
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   while ( $line = <FILE> ) {
        
      push(@outLines, $line); #to be sent to appended large file
   }
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No need for a loop there, you could just do:

   push @outLines, <FILE>; #to be sent to appended large file
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