#! /usr/bin/perl -w
use 5.008;
use strict;
use IO::Select;

# program and args
my @work = qw(sleep 3);
my $wanted = 2;

my $s = IO::Select->new;
my $have = 0;

while ($wanted) {
    while ($have < $wanted) {
        open(my $fh, "|-", @work) || die "Could not fork: $!";
        $s->add($fh);
        $have++;
    }
    for ($s->can_read()) {
        $s->remove($_);
        $have--;
        close $_;
        die "Unexpected returncode $?" if $?;
        # Real code may change $wanted here
    }
}