The thing you are missing is do you really need to use $xyz outside of the scope of the subroutine. I would read up on variable scope in the Llama book and/or the Camel book. Basically, you can substitute the following:

&example(15);

sub example {
  my($xyz) = shift || 0;
  my($a) = $xyz + 49;
  print $xyz;
  print $a
}
[download]

After doing this, $a and $xyz won't be available to you outside of the subroutine. This is all assuming you're operating under strict. Really, you ought to read the Llama book if nothing else, it's fantastic information and explains a lot of this much more clearly than I can. =]

-marius