It sounds like you want to use fork() to launch each of the four processes -- something like this (untested): update: Based on ikegami's more knowledgeable reply, the following is the sort of thing you'd do if you weren't using Windows; but if you replace the fork stuff with your original "system 1,...", I think the while loop using "wait" will still do what you want. 
my %children;

for my $n ( 1..4 )
{
    my $child = fork;
    if ( !defined $child) { die "fork failed on $n: $!" }
    elsif ( $child == 0 )
    {    # this is what the child does:
        exec 'some', 'process', $n;
    }
    $children{$child} = $n;
}
while ( keys %children )
{
    my $reaped_pid = wait;
    if ( exists( $children{$reaped_pid} )) {
        checkerror_function( $children{$reaped_pid} );
        delete $children{$reaped_pid};
    }
    else {
        # if you get here, better study "perldoc perlipc"
    }
}

[download]
I would assume that the "checkerror" function needs to know which of the sub-processes has just finished, so that it knows which set of output to check. That's why I kept track of the loop counter when forking, and pass it to checkerror_function when reaping.

(update: fixed missing close-quote and added slight improvement to "die" message)

In reply to Re: question about running a system and a perl function background by graff
in thread question about running a system and a perl function background by Anonymous Monk

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