Looking at the pod, Crypt::Blowfish takes any random string up to 56 bytes (i.e. 448 bits) long for its key, e.g. my $key = pack("H16", "0123456789ABCDEF"); All you need to do is string together 56 random bytes. What you're really after is a true random number generator, because the default rand is insufficient for cryptographic purposes. If you're on a Unix system with a decent /dev/random device (look at your system docs), you can use the Crypt::Random module:

use Crypt::Random qw( makerandom ); 
my $r = makerandom ( Size => 448, Strength => 1 );
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Another alternative which isn't quite as easy to use for cryptography is Math::TrulyRandom, but it's slow (though one 56-byte key won't kill you, certainly). There's some discussion here regarding cryptography and random numbers as well.