It kinda looks like you have copied code from somewhere else without much understanding and poured it into an editor. Reading and understanding other code is a good way to learn. Copying without understanding is a good way to get frustrated. Lets see if we can tip the balance toward understanding and introduce some good habits along the way.

1. First off, use strict is good! You should use warnings rather than adding the -w switch though.
2. Don't declare variable until you need them. Both @lines and $ave are declared out of context.
3. If you are looking for a file use -f instead of -e. (See -X documentation.)
4. Mega points for three parameter open and checking the result. Bonus points if you use lexical file handles and show the file name in the die: open my $fIn, '<', $infile or die "Can't open '$inFile': $!\n"
5. What do you understand chomp (@lines = <FIN>); to do?
6. What do you expect while (<STDIN>) {...} do to?
7. What value do you expect average to see in average(<FIN>);
8. Always use an explicit return statement if a sub is expected to return a result.