Please read Markup in the Monastery for advice on formatting.

Please read How do I post a question effectively? and/or How do I post a question effectively? and/or I know what I mean. Why don't you? for more information on what constitutes a good-faith effort to post a question -- which is tantamount to asking others to spend their time providing that help.

Update: And come to think of it, I'm not sure how much stock I'd put in offerings from a website that misspells "available" nor offers any insight into how the information is obtained. IMO, http://rscript.org/lookup.php?type=namecheck&name=foo.com was far-too-fast to have done any realtime checking with registrars, whois, or anything much else.

Good catch in your update - I fed it a domain I own and got:

START
NAMECHECK: AVALIBLE
END
[download]

(moments later) Hitting the URL without any parameters - http://rscript.org/lookup.php - yields

START
ERROR: ?type= stats || rank || top10 || players || rscplayers || rskb 
+|| ge || gers || geupdate || getop || bash || qdb ||
youtubeinfo || ytinfo || urban || halo3 || reach || activity || track 
+|| trackrank || trecord || tcompare || gcalc || weather ||
wowitem || wow || clan || rsforums || spell || namecheck || loginnamec
+heck || rsnews || alog || bing || ipdns || drops || wp ||
questxp || fml || mlia || tfln || translate please.
ERROR: This script is partially by micksam7, or at least maintained ev
+ery so often. In any case: Please don't eat the fish.
NOTICE: Please don't flood this script or use for purposes against Run
+eScape Rules. Your ip is sent in the user-agent and x-forwarded-for o
+f our script.
NOTICE: It'd be really cool if you acknowledge RScript if you use thes
+e scripts <3 Really really cool <333333
END
[download]

So it's not a domain availability check; player-name would be my guess.

Your code, fixed:

#!/usr/bin/perl -w
use strict;
use warnings;
use LWP::Simple;

my $data_file="common.txt";
open DAT, '<', $data_file || die "Could not open file!";
my @raw_data=<DAT>;
close(DAT);

# set once outside loop, not each time around
my $url = 'http://rscript.org/lookup.php?type=namecheck&name=';
foreach my $name (@raw_data) {
  chomp $name;
  #my $name = $name; # ??? set above
  #print "$url$name\r\n";
  #my $content = ("GET", '$url$name'); get is a function from LWP::Sim
+ple
  #my $content = ("GET", '$url$name'); $url$name in single quotes is t
+hat exact text
  my $content = get("$url$name");
  die "Couldn't get $url$name" unless defined $content;

  if($content =~ "NAMECHECK: NOTAVALIBLE") {
    print "\"$name\" is niet beschikbaar\n";
  }

  if($content =~ "NAMECHECK: AVALIBLE") {
    print "\"$name\" is beschikbaar\n";
  }
}
__END__
"jj" is niet beschikbaar
"jakejake" is niet beschikbaar
"blowmeuptom" is beschikbaar
"bilderberger" is niet beschikbaar
"rothchild" is niet beschikbaar
"ronaldmcdonald" is niet beschikbaar
[download]

Thanks for the help everyone. It's indeed a player name checker and not a domain checker. I'll try out the edited code someone posted. Thanks!