These can be named anything you want for the most part; using letters, digits and under-scores (can't start with digit). However, there are some common ones that Perl has created. These are STDIN, STDOUT, STDERR, DATA, ARGV, ARGVOUT. This will detail the main three though.

filehandles.txt

\!h STDIN & STDOUT - the barebones
STDIN  - your program should blindy read from this (input).
STDOUT - and blindly write to this (output).
$ ./your_program <dino >wilma # shell prompt command
The program's input should be read from the file 'dino' amd the output should go to the file 'wilma'

\!h STDERR 
This can be used to redirect any errors to wherever the user desires, though by default they go to that user's display screen.
Sending the errors to a file with a shell command:
$ netstat | ./your_program 2>/tmp/my_errors