Bug #73278 for Medical-NHSNumber:

Try the following code: my $silly = '12345'; my @is_valid = Medical::NHSNumber::is_valid($silly); if (@is_valid) { print "$silly is valid!\n"; } It reports that the number is valid. This is because the is_valid function, when called in list context, returns a list containing one item (undef). Any list containing one item evaluates to true, not false. The code above is quite contrived, but there are situations when you can induce list context without really meaning to. The solution is quite simple: rather than returning undef, just return nothing. That is, replace things like this: return undef unless ( scalar(@digits) == 10 ); with things like this: return unless ( scalar(@digits) == 10 ); A bare "return" like that will always be treated as false whether called in scalar context or list context.