Personally I try to avoid the use of signals in cases like this because they are dangerous in old perls and hard to get racefree in new perls (you need to take care of the case that a process exits before you properly stored the pid).

Instead of that I usually try to have a pipe into the processes and poll/select on them. Most programs don't close the standard handles, so you can actually use these. In this case I use STDIN.

The following code demonstrates the idea. Modify it to fit whatever your constraints are.

#! /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
    }
}
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