When undef is treated as a number, it's equal to zero. When it's treated as a string, it's equal to ''. 

$ perl -le 'print "true" if undef == 0;'
true
$ perl -le 'print "true" if undef eq "";'
true

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Checking to make sure the value is defined before comparing to zero is the right way to go.

In reply to Re: hashref value comparison by kyle
in thread hashref value comparison by geektron

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