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Not paying close attention to Perl’s definition of truth can sometimes lead to subtle bugs. Consider a simple scalar $x that should contain a string exactly one character wide. If the original value of $x can be undefined and you want to make sure it has a default value of a single space, do not do the following:

$x ||= ' ';


Why not? If $x starts off as ‘0′, a permitted value, this line will change it to ' '. Instead, do this

$x = ' ' unless defined $x;


Remember, 0, '0', '', and undef all evaluate to Perl’s notion of false.

The What is truth? by Galen Charlton, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Share Alike 3.0 United States License.