I'm not sure why there's a difference -- I don't have a windows machine to test it on, but I would have expected the "attempt to modify readonly value" error everywhere.

Keep in mind that you can directly modify the parameters passed to a function by messing with @_. You don't need to use \shift; I suggest rewriting modify_string as:

sub modify_string {
  $_[0] =~ s/u/i/;
  return $_[0];
}
[download]

Your way isn't "wrong", but (relatively) it does a lot of extra work.

hdp.