#!/usr/bin/perl

use strict;
use warnings;

my $ping = "/bin/ping";

while (<DATA>) {
    chomp;
    my $ping_out = `$ping $_ 2> /dev/null`;
    chomp ($ping_out);
    if ($ping_out !~ /bytes from/) {
        print "$_ isn't pinging\n";
    } else {
        print "$_ is up\n";
    }

#  <--- You're missing a '}' right here to end the while { } loop

__DATA__
SERVER01
SERVER02