You can pack a null-padded ASCII string using the Z pack template:

$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__
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And the testing section prints the expected: \x00\x00\x00\x0dHello, world!\x00\x00\x00 Since you're packing the unpadded length in the first four bytes, you'll have to make it the next multiple of 4 before unpacking the string as well.