Motivation for this change

The MailSPF Perl module can't be included as a library in other Perl modules because it puts library code in $out/lib/perl5 instead of $out/lib/perl5/site_perl.

I'm not familiar with Perl and the way Nixpkgs works with Perl, but basically, without this patch, having MailSPF in buildInputs doesn't add it to the PERL5LIB environment variable, while all other Perl dependencies in buildInputs that install library code to $out/lib/perl5/site_perl are automatically added to PERL5LIB. After this patch, MailSPF is properly added to PERL5LIB.

Things done

Force the install library path to be $out/lib/perl5/site_perl.