File input and output is not much of a stretch from normal I/O. Basically you have to use the open command to open a filestream and then read to and write from it. Then once you're done with it you use close to close the file. The syntax for opening a file is:

open(FILEHANDLE,"filename");

This opens a new filehandle with the name of FILEHANDLE, and associates it with filename which is the location of the file on your disk. This works for reading a file. If you want to write to it you need to put a > in front of the filename as seen below:

open(FILEHANDLE,">filename");

To append to a file you use >> in front of the filename as you can see here:

open(FILEHANDLE,">>filename");

Now for some quick examples:

open(FILE, "data.txt");          #opens data.txt in read-mode
while(<FILE>){                   #reads line by line from FILE which i
+s the filehandle for data.txt
   chomp;
   print "Saw $_ in data.txt\n"; #shows you what we have read
}
close FILE;                      #close the file.
[download]

To print stuff to a file you merely through a line into your program like:

print FILEHANDLE "your text here\n";

FILEHANDLE needs to be a FILEHANDLE you currently have open. Notice there is no comma between the FILEHANDLE and the stuff your printing out. That is the way it needs to be. Now for a more extended example:

open FILE, ">keylogger.dat";  #opens file to be written to
while(<>){                    #while we're getting input from the keyb
+oard
   print FILE $_;             #write it to our file
}
close FILE;                   #then close our file.
[download]

Another thing you might want to look into are the filetest operators these allow you to do things like test whether a file exists, is readable, is writable among other things. complete information look at the documentation