As for the original post, perhaps you're testing for the wrong phenomenon.

#!/usr/bin/perl
use strict;

sub foo {
  my($bar) = @_;
  print "foo: $bar\n";
  ++$bar;
}

foo(1);
foo();
foo();
foo();
foo(1);
foo();
foo();
foo();
[download]

Produces:

foo: 1
foo:
foo:
foo:
foo: 1
foo:
foo:
foo:
[download]

So the empty @_ does reset the var, which was I think your original assertion.

Matt

Update: Elian is right. That's what I get for posting pre-coffee. His original point involved a conditional assignment. I suppose if there's a moral to this mini-thread it is to make your parameter assignments mandatory unless you really are intending to keep old values around. :-(