#!/usr/bin/perl
use warnings;
use strict;

my @commands = ( 'sleep 3 && find /usr/share/doc',
                 'sleep 4 && find /opt',
                 'sleep 5 && df' );

my @pids;
for my $i (0 .. $#commands) {
    if (my $pid = fork) {
        push @pids, $pid;

    } elsif (defined $pid) {
        0 == system "$commands[$i] > $i.o" or die "Can't run $commands[$i]";
        exit

    } else {
        die "Can't fork";
    }
}

for my $i (0 .. $#commands) {
    print "=== $commands[$i] ===\n";
    waitpid $pids[$i], 0;
    open my $fh, '<', "$i.o" or die "$i.o: $!";
    print while <$fh>;
    unlink "$i.o";
}