Crypt::Twofish_PP::Key16 is a Perl module with Twofish with 16 byte (128 bits) keysize.
my $cipher = Crypt::CBC->new (key => 'my secret key',
cipher => 'Twofish_PP::Key16');
This module is only a helper module and you should never use it directly. Use Crypt::Twofish_PP(3) instead and see there for more documentation.
The standard module for Cipher Block Chaining (CBC) in Perl, Crypt::CBC cannot grok with variable key sizes. However, the Twofish algorithm is defined for key sizes of 16, 24, and 32 bytes, but there is no way to communicate that to Crypt::CBC.
If you want to use Crypt::Twofish_PP in CBC mode with a keysize of 16, simply specify Crypt::Twofish_PP::Key16 as the algorithm. It is eqeuivalent to Crypt::Twofish_PP but it will report a default keysize of 16 bytes back to Crypt::CBC.
Note that this is not necessarily the real keysize. The method keysize() of Crypt::Twofish_PP only exists to satisfy Crypt::CBC. The module will derive the real keysize from the length of the key you supply.