spin($j) increments $j by editing its $_[0]. As perlvar says,
A function is free to do in-place modifications of @_ and change its caller's values. 
upcase_in($v1, $v2);  # this changes $v1 and $v2
sub upcase_in {
 for (@_) { tr/a-z/A-Z/ }
}

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Scalars are already passed by reference, so you can modify scalar arguments without using typeglob mechanism (skipped) by referring explicitly to $_[0] etc.
Sorry if my advice was wrong.

In reply to Re^5: Spinning cursor while waiting for search/copy process to be finished by aitap
in thread Spinning cursor while waiting for search/copy process to be finished by Ingvar

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