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;

##</code><code>##

use LineDebug;
# code...

##</code><code>##

use LineDebug freq => 10;
# code...