I'd do it with threads this way. (I've used ping cos I don't have nmap but that's a one line change).

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

if( !@ARGV or $ARGV[ 0 ] !~ m[\d{1,3}\.\d{1,3}\.\d{1,3}] ) {
    die "Usage: $0 xxx.xxx.xxx\nEnter 3 octets only\n";
}

my $dom      = $ARGV[ 0 ];
my $Qwork    = new Thread::Queue;
my $Qresults = new Thread::Queue;

sub work{
    yield until $Qwork->pending;
    while( $Qwork->pending ) {
        my $sub = $Qwork->dequeue;
        system "ping -n 1 -l 0 $dom.$sub 2>&1 1>nul";
        $Qresults->enqueue( 
            sprintf "$dom.$sub %s alive", $? >> 8 ? 'is not' : 'is' 
        );
    }
    return;
}

threads->new( \&work )->detach for 1 .. 20;
$Qwork->enqueue( 1..255 );
yield until $Qresults->pending == 255;

print $Qresults->dequeue while $Qresults->pending;
[download]