#!/usr/bin/env perl
use warnings; use strict;
use 5.012;
use IPC::System::Simple qw(system);

system( 'xterm', '-geometry', '80x25-5-5', '-bg', 'green', '&' );

say "Hello";
say "World";

system("cmd &")

$SIG{CHLD} = sub { wait };

$SIG{CHLD} = 'IGNORE';

unless ($pid = fork) {
    unless (fork) {
        exec "what you really wanna do";
        die "exec failed!";
    }
    exit 0;
}
waitpid($pid, 0);

system('xterm -geometry 80x25-5-5 -bg green &');

if (my $pid = fork) { #exits 0 = false for child process, at this point is brain split
  # parent ($pid is process id of child)
  #do something what you want, asynchronously with executed command
  waitpid($pid);  #wait until child ends
  #if you don't want, don't wait. you process ends, then child process will be relinked
  #from your script to INIT process, and finally INIT will assume child finishing.
  #alternatively you can handle SIGCHLD signal in your script
} else {
  # child
  exec('some_command arg1 arg2'); #or exec('some_command','arg1','arg2');
  #exit is not needed, because exec completly overwrites process content
}