BrowserUk,
While you already have a solution that works for your specific needs, I have come up with several others in the past:

while (my $next = $iter->()) {
    # ...
    last if $iter->('exhausted');
}
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The above works when your iterator is designed to be called in a void contextwithout parameters. Even when it is not, you can come up some argument that is assumed to never be a valid input - that is of course, until there is a situation like you have where any valid scalar value is valid input, then you might do something like:

while (my $next = $iter{next}->()) {
    # ...
    last if $iter{exhausted}->();
}
[download]

The above may look ugly at first, but it allows possibilities like reversing the direction of the iteration, peeking at the next value without advancing the iterator, going back to the previous value, "redo"ing, etc. I have very seldom had a need for it but there it is.