T'is easy with threads:

Updated: Changed < to <= and made final loop wait for all kids to finish.

#! perl -slw
use strict;
use threads qw[ yield ];
use threads::shared;

our $MAXTIME    ||= 7200;
our $CMDS       ||= 140;
our $CONCURRENT ||= 6;

my $running :shared = 0;

my @cmds = map {
    my $time = int rand $MAXTIME;
    qq[ perl -e"sleep $time" ];
} 1 .. $CMDS;


for my $cmd ( @cmds ) {
    sleep 1 while do{ lock $running; $running >= $CONCURRENT };
    async{
        my $n = do{ lock $running; ++$running; };
        my $tid = threads->tid;
        print "$tid: starting '$cmd' ($n)";
        system $cmd;
        print "$tid: ending";
        { lock $running; --$running; }
    }->detach;
    yield;
}

sleep 1 while do{ lock $running; $running };
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Sample run: