Along the sames lines, and much more likely, is to pass in a numeric option on the command line whose value is quite probably 0. For example, I have a histogram script in which I can pass in the minimum value. Unless I check the value using defined, the thing just doesn't work right:

use strict;
use warnings;

use Getopt::Std;
our $opt_m;
getopts('m:');
print "$opt_m\n" if ($opt_m);           # Wrong
#print "$opt_m\n" if (defined $opt_m);  # Right
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