Maybe there's no mail waiting there for you ? In that case $pop->Count() will be 0.
You should check to see whether the connection succeeded. The documentation says:
To test for a connection failure, you will need to check the number of
+ messages: -1 indicates a connection error.
For USESSL to be successful, the documentation also says that you need to have IO::Socket::SSL installed. Do you have that module ?
I would expect that you'd get an error if you don't have that module installed .... however, the documentation suggests that the absence of that module will simply cause the connection to silently fail.