I suspect you'd have to use a source filter to get the behavior that you're looking for, and then you run the risk of behavior you're not looking for.

The next best thing would be to document that your iter() won't work if called twice from the same source line. I'd want to reformat the line you have anyway. If you're so inclined, you could allow an optional third parameter in which the programmer has to write a unique name (or gamble on rand) to distinguish calls on the same line.

Ultimately, I'd say, don't do this. It makes more sense for me, the maintenance programmer, to have an "initialization" of the iterator somewhere followed by multiple calls to the iterator to get its results.

Want to mess with the mind of the reader? Make this work:

for( my $x = iter(1,10); $x; $x++ ) {
    print "$x\n";
}
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(Make iter() return an object that's overloaded with stringification and numification to the "current" value and "++" that proceeds to the next value. You might even be able to do this with only tie.)