$str = 'Hello, world!';
$len = length $str;

$packed_str = pack 'N', $len;

$len += 4 - (($len % 4) || 4);       # next multiple of 4

$packed_str .= pack "Z$len", $str;

# for testing:
($test_str = $packed_str) =~ s/([^\x20-\x7E])/sprintf '\\x%02x', ord $1/ge;
print "$test_str\n";

__END__