You have text you need to serialize into bytes. The process is called encoding.

print $socket encode('UTF-8', $text);
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Or if the only thing you transmit on the socket is text, you could use binmode to add an :encoding layer.

Are there any circumstances where pack('a*', $payload) does not give the same value was $payload?

Since you're talking about internal storage formats, yes.

$_=5;             # IV    -> PV
$_="abc"; s/.//;  # OOK=1 -> OOK=0
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Why is the OO binmode method available for IO::File but not IO::Handle? Is it not applicable to all handles?

Weird. It makes no sense to me either. binmode doesn't apply to Dir handles, but IO::Handle is not a base class of IO::Dir, and the other methods of IO::Handle don't apply to directory handles either.