Others have mentioned that PREFIX and --install_base work differently, and you probably don't want to combine them. You should probably change PREFIX to INSTALL_BASE.
The direct problem you are having is because you've configured the --install_base option on the mbuild_arg, which is the build step. It needs to be set either on the install step (mbuild_install_arg) or the configure step (mbuildpl_arg). Setting it on mbuildpl_arg would correspond to how you were using the option manually.
I would definitely recommend using local::lib instead of configuring this in your CPAN shell. And if you didn't want to use local::lib, you would be better off using the PERL_MM_OPT and PERL_MB_OPT environment variables to change the installation location. Either local::lib or using the environment variables manually has the advantage of working with any install tool, not just the CPAN shell.
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