unpack with "b*" could be a solution:

use strict;

my $a = "1234".chr(4000)."abcd";
{
        use bytes;
        print length($a), "\n";  #11 bytes
}

print length($a), "\n";   #9 chars
print length(unpack("b*", $a)) / 8; # 11 bytes
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I tried with 5.8 seems fine. However, I leave two things open:

1. whether unpack b* is supported in your old versions? (I cannot test, as I don't have those old versions)
2. for long strings, I guess performance would be an issue.