Actually, what worries you is not Perl per se, but people that write Perl code and don't know what they want to test for. The code shown interrogates $lastname for values that represent truth in Perl, while it ought to be checking for definedness:
if(defined $lastname) { ... }
The two are totally different cases. I would also argue that the problem lies elsewhere if you have values for a 'lastname' field in your data set that consist of a single letter.
Considering that there a plenty of people with no last name at all, I don't find it at all hard to imagine that there might also be people with a real last name consisting of only one letter.
There is a town famously called simply "Y" in France.
