for (my $x = 0; $x < $total_number_runs ; $x++) {
    exec("shell_script < input_file");
}

##</code><code>##

for my $x (1..$total_number_of_runs) {
    my $pid = fork;
    if (not defined $pid) {
        die("Unable to create child process: $!\n");
    }
    exec("shell_script < $input_file");
}