It is indeed possible to set $! in user code.

perl -e 'print ("$_ gives ", ($!=$_), "\n") for 1..20'

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However you will note that its behaviour is highly magical and (unless you do some magic of your own) not particularly usable for any purpose other than its intended.

In reply to Re (tilly) 2: An example of programming by contract by tilly
in thread An example of programming by contract by princepawn

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