Crypt::Twofish_PP::Key24 is a Perl module that offers a Twofish with 24 byte (192 bits) keysize.
my $cipher = Crypt::CBC->new (key => 'my secret key',
cipher => 'Twofish_PP::Key24');
This module is only a helper module and you should never use it directly. Use Crypt::Twofish_PP 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 24, 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 24, simply specify Crypt::Twofish_PP::Key24 as the algorithm. It is eqeuivalent to Crypt::Twofish_PP but it will report a default keysize of 24 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.