use POSIX qw/setsid/;

$pid = fork();
if (!$pid) {
   setsid;                 # set our pgid to $$
   system("sleep 30");     # just an example, put your xterm here
} elsif (defined($pid)) {
   $SIG{ALRM} = sub { kill 'TERM', -$pid };
   alarm 3;
   # .. do some stuff that will take longer than 3 seconds
}