A port number is a way of identifying a service on a machine. There are default port numbers, like 25 for SMTP, 23 for Telnet, 22 for SSH, 80 for HTTP, 443 for SSL, etcetera, but they can be changed. Only services offering something on a network (even if it is itself) need port numbers, so perl in itself doesn't have a port number.
One way to look at port address is this analogy: The IP# is the Zip/Postal code for your town, the port number is your house. I know, it's not the best analogy, but it works. . .
Update: Fletch is correct, 443 is for HTTP over SSL.
FTP uses 20 & 21, POP3 uses 110, DNS uses 53, finger uses 79, and my favorite, gopher, uses 70. For a listing of port assignments, look here.
