You didn't describe what your problem was. What happens? What's the error? It will help if you add

use warnings;
use strict;
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to the start of your code. This will enable warnings and force you to declare your variables, which greatly aids in catching mistakes.

Now, on the topic of your code: You're trying to send an email with one very long list of recipients. It's possible your email server will not honor this. You may want to try sending one email/address, like so (code UNTESTED):

use strict;
use warnings;

# Define constants
my $address_file = "email.txt";
my $message_text = "message.txt";
my $from_addr = "anywhere@home.com";
my $mailhost = "localhost";

# Always check to see if file opened or not
open(MSG, "$message_text") || die "Could not open $message_text! $!\n"
+;

my $subject;
my $body;

LINE: while(<MSG>) {
     if /^subject:(.*)/ { $subject = $1 and next LINE};
     $body .= $_;
}
close(MSG);

open(LIST, "$address_file") || die "Could not open $address_file! $!\n
+";

# Send an email for each address in the file
# Note we're not doing any validation on addresses
while(<LIST>) {
     chomp;
     
     my $sender = Net::SMTP->new("$localhost"); 
     $sender->mail("$from_addr"); 
     $sender->to("$_"); 
     $sender->data(); 
     $sender->datasend ("From: Home\n");   
     $sender->datasend ("Subject: $subject\n\n"); 
     $sender->datasend ("$body\n");
     $sender->dataend(); 
     $sender->quit(); 

}
close(LIST);
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Hope this helps. Cheers,
ibanix

$ echo '$0 & $0 &' > foo; chmod a+x foo; foo;