Monthly Archives: September 2009

What is truth?

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.