This seems like a straightforward regex job:

my $text_b="cac0bde7c3b3d2d7d7e9d6af0a";
$text_b=~s/([0-9a-f]{2})/\\x\1/g;
print $text_b;
__END__
\xca\xc0\xbd\xe7\xc3\xb3\xd2\xd7\xd7\xe9\xd6\xaf\x0a
[download]

Update: Oops - seems I misunderstood the meaning of "encoded literal string". Never mind my rambling (or downvote it :-) ) - pack as above is the solution.

CU
Robartes-