open(NPBXNUM1, ">npbxnum1");
open(MYINPUTFILE, "npbxnum");

while (<MYINPUTFILE>) {
   my($line) = $_;
   chomp($line);
            
   foreach my $inputData ($line) {
            
      my $outputData = $inputData;
      while (length($outputData) < 4) {
         $outputData .= '_';
      }
   print NPBXNUM1 "$outputData\n";
   }
}
close(MYINPUTFILE);
close(NPBXNUM1);
[download]

You're welcome, again -- are you up for a little code analysis? There are a few things in the script as you've modified it which really ought to be adjusted if you have the time.

1.   open(NPBXNUM1, ">npbxnum1");
     open(MYINPUTFILE, "npbxnum");
   [download]

   You went through the effort of specifying > on the output file; consistency has much value in engineering. Why not boldly include the < on the input file?

     open(NPBXNUM1, ">npbxnum1");
     open(MYINPUTFILE, "<npbxnum");
   [download]

   And I'm sure other Monks would strongly recommend breaking the < and > out into their own parameters, for reasons I don't pretend to understand:

     open(NPBXNUM1, '>', "npbxnum1");
     open(MYINPUTFILE, '<', "npbxnum");
   [download]

2. while (<MYINPUTFILE>) {
      my($line) = $_;
      chomp($line);
   [download]

   probably better as:

   while (my $line = <MYINPUTFILE>) {
      chomp($line);
   [download]

3. foreach my $inputData ($line) {
     my $outputData = $inputData;
     while (length($outputData) < 4) {
       $outputData .= '_';
     }
     print NPBXNUM1 "$outputData\n";
   }
   [download]

   The foreach loop is pointless, as its purpose is to iterate through an array; $line is a scalar.

   Perhaps better as:

     my $outputData = $line;
     while (length($outputData) < 4) {
       $outputData .= '_';
     }
     print NPBXNUM1 "$outputData\n";
   }
   [download]