Not quite. There are three bugs in your code:

• Incorrect results are returned when the most significant bit is set. A sign bit should be added, adding an extra byte. *
  "FFFF" should return ("\x00", "\xFF", "\xFF").

• Incorrect results are returned when there are leading zeroes.
  "000102" should return ("\x01", "\x02").

• Incorrect results are returned when an odd number of hex digits are provided.
  "102" should return ("\x01", "\x02").

Relevant docs:
java.Math.BigInteger.BigInteger(java.lang.String, int)
java.Math.BigInteger.toByteArray()
java.Math.BigInteger.bitLength()

Fixed code:

my $key = "0123456789ABCDEF0123456789ABCDEF";

local $_ = $key;
s/^0+//;
$_ = "0$_" if length % 2 == 1;
$_ = "00$_" if /^[89ABCDEFabcdef]/;
my @b = map { chr hex $_ } /(..)/g;
[download]

* – I'm assuming new BigInteger("FFFF", 16) is considered to be 65535 (not -1).

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