Yes, I agree and had noted the OP's new message. In fact, I would consider this:

sub diag {
  my ($x, $y) = @_;
  sqrt( $x^^2 + $y^^2 );
}
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to still be very fine (perhaps slightly slower, but very clear), but the way the original script was coded with numerous intermediate variables (to store the squares, etc.) actually made the things more complicated to follow than a single and simple math expression which all of us presumably can understand at first glance.