Just to make a bad idea, slightly better....
At least make a sub for printing to stderr. How about adding:

sub debuglogger {
  my $msg = shift;
  if ($DEBUG) {
    print STDERR "$msg\n";
  }
}
[download]

Then changing your globlal replace command to:

's/sub\s+(\w+)\s+{/sub $1 { debuglogger("entering $1"); /'
[download]

That way you can toggle the output with the $DEBUG var, and do fancy things like adding timestamps or changing where to debugging information goes w/o resorting to another -pi -e command.

-Blake