In old version of perl (before 5.6) we could not use scalar variables as file handles so we used uppercase letters such as XYZ or INPUT, QQRQ or FILEHANDLE.

Also the function had only 2 parameters.

use strict;
use warnings;

my $filename = "data.txt";

open(FH, ">$filename") or die;
print FH "data";
close FH;

open(FH, $filename) or die;
my $line = <FH>;
close FH;

Security problems.

Being global, difficult to pass as parameter to functions.

Don't Open Files in the old way