Rendered readable:

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1135034

use strict;
$| = 1;

my $active_readers = 0;
my $current_reader_limit = 10;
my $total = 0;

while(1) {
    if($active_readers < $current_reader_limit) {
        $active_readers++;
        ++$total;
        if(my $pid = fork) { # parent
            print "Spawned child $active_readers pid $pid  total $tota
+l\n";
        }
        elsif(defined $pid) { # child
            exec "echo Hello from child $active_readers pid \$\$"
            or die "exec failed with $!";
        }
        else { # fork failed
            die "fork failed with $!";
        }
    }
    elsif((my $pid = wait()) > 0) {
        $active_readers--;
        print "Reaped $pid, active = $active_readers\n"
    }
}
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