Re^5: Summing the odd/even digits of a larger number (for upc check digits)...a better way?

You are right that undef is not being returned. False is being returned. False is a dual var. It's 0 when used in numerical context, and '' when used in string context. That's not quite the same as '', as the following shows:

use warnings;

my $false  = !1;
my $nulstr = '';

print("'$false'\n");     # ''
print("'$nulstr'\n");    # '' 

print(0+$false, "\n");   # 0
print(0+$nulstr, "\n");  # 0 with 'Argument "" isn't numeric'
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