Note that crypt and getpw* won't work because some systems store passwords in /etc/shadow, and/or use MD5 to encrypt them (and the crypt function doesn't behave accordingly). Sometimes passwords might be managed off machine.

You'll find the Authen::PAM module helpful if your system supports PAM (Pluggable Authentication Modules). Otherwise, Crypt::PasswdMD5 might be useful, as well as looking up the documentation for what fields are in /etc/shadow.

If your system uses shadow passwords it is likely that your script will need to have root access. Regardless, you have a serious security issue if people can try to brute-force passwords on your system through your script.