That approach is going to fail on any system that uses shadow password files.
I don't know why. getpwent() and friends do the right thing even in systems with shadow passwords, provided you are superuser.
Perl's crypt() will call whatever crypt() is there in the system's library. Crypt::Passwd and Crypt::PasswdMD5 will cover the gap if you want to use a file from a different (incompatible) crypted password, but this is not the case at hand..