Ok, try this. It's not guaranteed to work, but it should be better than trying to brave the multi-line problem from above.

package LineDebug;

$LOADED = 0;
$insert = qq<print "\\tDEBUG: " . __LINE__ . "\\n";\n>;

sub import {
  return if $LOADED++;
  my $freq = (@_ == 3 && pop) || 1;
  my ($file) = (caller)[1];
  my $code;

  open PROG, $file;
  while (<PROG>) {
    $code .= $_;
    if ($freq == 1) {
      next unless /\S/;
      $code =~ s/;\s*\n\z/;  $insert/;
    }
    elsif ($. % $freq == 0) {
      if (!/\S/) { $code =~ s/\n\z/;  $insert/ }
      else { $code =~ s/:;\s*\n\z/;  $insert/ } 
    }
  }
  close PROG;

  # print $code;
  eval $code;
  exit;
}

1;
[download]

It's used like so:

use LineDebug;
# code...
[download]

or

use LineDebug freq => 10;
# code...
[download]

It will print the line number of the file on every line ending with a semicolon (THIS IS NOT 100% ACCURATE). Specifically, it will not do this on blank lines. If you give it a frequency argument, it will print every X lines, if that line ends in a semicolon or is blank.

$_="goto+F.print+chop;\n=yhpaj";F1:eval