I'd do it like this:

#!/usr/bin/perl

use strict;
use warnings;

my @hosts;

while(<>){
    chomp;
    die "usage: do_something [file]\n"
        if $ARGV eq '-' and ! $_;
    next unless $_;
    push @hosts, $_;
}   

print "Read hosts:\n";
print $_,$/ for @hosts;
[download]

That will bail at an empty input line from STDIN. Works with

do_something hosts
do_something < hosts
echo foo | do_something
do_something
type
type
type
[download]