so for using that "sh Configure -Dprefix=~/p-5.18.0/" I have to re-install perl?

Yes

See , you're not supposed to mess with your system perl, you're just supposed to use system tools to add modules to it (yum, apt ...)

So you install a second/additional "local" perl ... and then it knows where to install modules without needing those extra env vars

Root simply uses "/user/local/bin/cpan" to install modules into "/user/local/perl/..."

Root tells his users to use "/user/local/bin/perl"

All the users use "#!/user/local/bin/perl --" for shebang and things just work

Now if the users want extra stuff from cpan they use PERL5LIB/INSTALL_BASE... and they don't bother root about it or other users