Other monks have addressed the source of your warning messages, but here are a couple of other tips:

• If you are creating dynamic HTML, then you really should use the CGI module.
• You are checking the return value of open, which is good. However, the three argument form is generally recommended over the two argument form. So instead of:

  open(USERS, "< $userlist") or die "cannot open file: $!\n";
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  you should write something like:

  open USERS, '<', $userlist or die "Cannot open $userlist:$!\n";
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• Your first for loop:

  for (my $i = 0; $i < scalar(@users); $i++) {
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  could be written more "perlishly" (and more cleanly) as:

  for my $user ( @users ) {
  [download]

Hope this helps,
Darren :)