The above suggestions are good. If you are sure the command will be run in a terminal you could do

#!/usr/bin/perl

use strict;

my $cmd   = '/bin/who';  # or wherever it is on your system
my $clear = '/bin/clear';
my $sleep = '10' 

while (1) {
   system("$clear"); # clear the display
   system("$cmd");        # send the command to STDOUT
   sleep($sleep);         # sleep a bit
}
[download]

use perl;