You sound confused about filehandles, STDIN is nothing but a Perl filehandle which is a label that describes a connection to a file, Perl has many types of filehanldes, examples are the STDIN, STDOUT, STDERR, now the way these are different has to be obvious from their context after you read about them, there're technical differences as well that got to do with buffering for instance, And you can have your own filehandles introduced too.

If you wanna learn following the right path then you might want to invest time in studying and reflecting, if you rely only on environmental impulses to motivate you you may well be laying the bricks on top of each other before they're hardened enough yet before you complete building and it'd come falling down on its weight.

Anyhow here's a list that can be of use (just for starters):

• Opening and closing file handles
• Input and Output
• Filehandles

NB: filehandles are customary written in uppercase.
Good Luck and Happy Perl journey.