You have to learn about fork-ing and managing child processes. Here's some example code:

use POSIX qw(:sys_wait_h);
sub doit {
    my $pid = fork();
    unless (defined($pid)) { die "unable to fork: $!\n"; }
    if ($pid) {  # parent
        # wait for child to finish and display waiting message
        my $start = time;
        while (1) {
            print "time spent waiting: ", time - $start, "\n";
            last if (waitpid($pid, WNOHANG) > 0);
            sleep(2);
        }
        # child process has terminated
        # read results from file
        open(R, "<results-from-child-process");
        while (<R>) { ... process results... };
        close(R);
    } else { # child
        # perform time consuming computation here.
        ...
        # leave results in some file
        open(R, ">results-from-child-process");
        print R ...;
        close(R);
        exit(0);
    }
}
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