Perl's built-in logical operators, C<and>, C<or>, C<xor> and C<not>
support 2-value logic. This means that they always produce a result
which is either true or false. In fact perl sometimes returns 0 and
sometimes returns undef for false depending on the operator and the
order of the arguments. For "true" Perl generally returns the first
value that evaluated to true which turns out to be extremely useful
in practice. Given the choice Perl's built-in logical operators are
to be preferred -- but when you really want pure 2-degree logic or
3-degree logic or multi-degree logic they are available through
this module

WWW: http://search.cpan.org/dist/Math-Logic/