The first byte of $packed is the length, and the rest is the string. Just as if you had initialized $packed like so:

my $s = 'the quick brown fox';
$packed = pack 'Ca*', length $s, $s;
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To unpack it, leave out the '/'

my ($len, $val) = unpack 'Ca*', $packed;
print "$len:$value";
__END__
output:
19:the quick brown fox
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If it is a bug it's a documentation bug; the meaning is very unclear. Using the '/' is only documented in pack, not in unpack - but it behaves differently in unpack than pack. With 'C/a*' as the template, unpack returns that many bytes. IE, whatever 'C/' represents will be how long a string is returned. Change it to 5 when you unpack it and you'll just get "the q".

With pack, it will prepend the length of the item packed.

Update: I missed the point about scalar versus list context. That is weird - probably a bug! From the pack docs:

The *length-item* is not returned explicitly from "unpack".

In scalar context, the length-item is returned (using AS 5.6.1 build 633).