"...while is empty..."

It isn't ;-) From "Learning Perl":

"Another way to read input is with the diamond operator: <>. This is useful for making programs that work like standard Unix utilities, with respect to the invocation arguments (which we'll see in a moment). If you want to make a Perl program that can be used like the utilities cat, sed, awk, sort, grep, lpr, and many others, the diamond operator will be your friend. If you want to make anything else, the diamond operator probably won't help."

diamond.pl

#!/usr/bin/env perl 

while (<>) {print}
[download]

data.txt

foo
bar
nose
cuke
[download]

Run it: ./diamond.pl data.txt

Regards, Karl