Some functions set the $! error code, like 

open(FILE,"</this/file") or die($!);

How would I get my own sub to populate '$!'? Is it any printing to STDERR that ends up in '$!'- no.. right? (I searched the docs but this is throwing me off) Is the special variable '$!' only set by C libraries or something spooky like that?

How would I get the following to work? (uncommenting the line doesn't do what I would like).

for(qw(1 4 haha)){
 usenumbers($_) or die($!);
}

sub usenumbers {
 my $var = shift;

 $var=~/^\d+$/ or
  # ( $! = 'these were not digits' ) and 
  return;

 print "Yes, [$var] has numbers.";
 return 1;
}

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In reply to How do I return false and set an error in special variable $! by leocharre

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