I am a true believer of abstracting your code into subroutines, but for this it just seems like a waste of time. Also, why bother writing the data back out to the same file? Just log the results to a new file. Anyways, the problem you are having is that you are not splitting each line. Since you didn't specify which email address you are using, i'll have to assume you meant all of them. If you know exactly which ones you want, just use split with an array slice:

open (FILE, '<', 'prev.txt') or die "Can't open prev.txt\n";

while (<FILE>) {
   my @email = (split('\s',$_))[1..3];

   for (@email) {
       # send email
       # log results
   }
}
[download]

If you don't know which fields will be email addresses, you can bring Email::Valid along for the ride:

use Email::Valid;

open (FILE, '<', 'prev.txt') or die "Can't open prev.txt\n";

while (<FILE>) {
   my @stuff = split;

   for (@stuff) {
      next unless Email::Valid->address($_);
      # send email to $_
      # log results
   }
}
[download]

Finally, please consider using MIME::Lite to send the emails. It's really easy to use and it really rocks. :)