while (<HANDLE>) {
    last unless 1..21;
}
... rest of processing here ...
[download]

last unless 1..21;

Won't this do the opposite of what was intended and skip all but the first 21 lines of the file?

No... the while goes through 21 one times, then breaks out... (tho might lose line 22... 1..20 might be needed in this case... but once it exits the loop you either start a new loop or read the reast into an array or whatever, since your placeholder in the file is now at line 22. You see? But that way it isnt perfoming the line check after the 21st line.. not a big efficiency savings... but a bit.
                - Ant

<HANDLE> for 1..21;
...
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

You said:

Any reason you wouldn't use:

<HANDLE> for 1..21;
[download]

Yes. Try that with <ARGV> (aka <>) when there's only 15 lines in the files specified on the command line. After it hits EOF, it now starts reading unexpectedly from STDIN for the remaining 5 lines (after 1 undef for the eof), because the process starts over!

Moral: Always watch for an early EOF from a filehandle.

-- Randal L. Schwartz, Perl hacker