Perl's pack() and unpack() offer partial solutions: the "N" format delivers a good fit (4 byte uint in network byte-order), but "A/*" encodes a string without extending the byte string payload length up to a multiple of 4-bytes.

Have you tried something like

my $l = length($string);
$l += 3; $l >>= 2; $l <<= 2;
pack("a$l", $string);
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