You're confusing the concepts of "defined" versus "false", which are very different in Perl. The getpwnam document mentions

If the entry doesn't exist you get the undefined value.

Thus, you need a test as mirod has outlined above.

The same consideration applies anytime a value that represents falsehood in Perl (0 or the empty string) can be a legitimate return result from a function. One of my favorite Perl books, Effective Perl Programming has an excellent section on the difference.