use Mail::Send ();

my @users = qw( bob carol ted alice );

while( 1 ) {
  for my $user ( @users ) {
    my $msg = Mail::Send->new( To => $user, Subject => "So, whatcha do
+ing?" );
    my $fh = $msg->open;
    print $fh "Please let me know.\n"
    $fh->close();
  }
  sleep( 360 );
}
[download]

And just wait for the responses to come flying in . . .

Or read How (Not) To Ask A Question and post a more coherent question.

This depends on what you mean by "user activity", on Linux you can get user logins with for example, Sys::Utmp or via WMI on windows, process information you can get via Win32::Process::Info or Proc::ProcessTable on Unix for instance. It is unlikely however that you will find something that will provide this information for both, I would guess that your best bet would be to create a "service" on each different machine that uses the appropriate method for the OS and expose that over the network so that the information can be captured by a single program elsewhere.

Of course both platforms support SNMP which can possibly be configured to provide the information that you need, and can be accessed over the network.

/J\

As pointed out before, the term user activity is not very clear. On UNIX-like systems you could 'monitor' users who use a BASH shell e.g. by simply tailing the .bash_history