That is because you can easily use Perl5 modules in Raku.
use Scalar::Util:from<Perl5> <looks_like_number>;
say ?looks_like_number( '5.0' ); # True
Then you can migrate one module at a time.
You can even subclass Perl classes using this technology.
Basically you can use the old codebase to fill in the parts of the new codebase that you haven't transferred over yet.
By that same token you can transition from Python to Raku in much the same way. The module that handles that for Python isn't as featurefull as the one for Perl yet.
# load the interface module
my $b64 = base64::b64encode('ABCD');
# Buf:0x<51 55 4A 44 52 41 3D 3D>
# Raku wrapper around a native library
my $b64 = base64-encode('ABCD');
# Buf[uint8]:0x<51 55 4A 44 52 41 3D 3D>
my $b64 = encode_base64('ABCD');
# workaround for apparent missing feature in Inline::Ruby
my \Base64 = EVAL ｢Base64｣, :lang<Ruby>;
my $b64 = Base64.encode64('ABCD');
So if I had to I could transition from any of those other languages above to Raku at my leisure.
Not like Python2 to Python3 where it has to mostly be all or nothing.