OK, that explains it. (I had an incorrect mental model: I was thinking that <FH> always put the data somewhere, using $_ unless otherwise specified. I don't know how I came to think that, as in hindsight that would be clearly stupid. Oh, well--live and learn!)

So the correct fix to my program would is to change the inner loop from:

  for my $cnt (0 .. 3) {
     my $t=<DATA>;
     print $t;
  }
[download]

to

  for my $cnt (0 .. 3) {
     $_=<DATA>;
     print;
  }
[download]

so I can avoid unnecessary variables and clutter.

Thank you!

or

for my $cnt (0 .. 3) {
   print scalar(<DATA>);
}
[download]

Cool! And then perhaps:

while (<DATA>) {
   if ($. % 10 == 1) {
      print;
      print scalar(<DATA>) for(0..3);
   }
}
[download]

Hold up. How does one variable - two, if you did what I do and use a variable for the outer read as well - equate to clutter? I'd probably remove the unused $cnt if I was going to knock clutter out.

use Modern::Perl;

while (my $line = <DATA>) {

    if ( $. % 10 == 1 ) {
        print $line;
    }

    for (0 .. 3) {
        my $important_line = <DATA>;
        print $important_line;
    }

}
[download]

I realize this is a matter of personal aesthetics, of course. I was just a little confused how declaring and using a variable (and avoiding weird behavior from <DATA>) was better than declaring an unused variable and getting the weird behavior.

Using a variable is even required if you want to add error checking.

while (my $line = <DATA>) {

    if ( $. % 10 == 1 ) {
        print $line;
    }

    for (0 .. 3) {
        defined( my $important_line = <DATA> )
            or die("Premature EOF\n");

        print $important_line;
    }
}
[download]