The only way to test if something is undef is to see if it's not defined: 
if (not defined $x) {
  print "\$x is undef\n";
}

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Using undef in numeric context (like you did) makes it look like 0; using it as a string makes it look like "".

japhy -- Perl and Regex Hacker

In reply to Re: what is *undef* ? by japhy
in thread what is *undef* ? by kiseok7

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