These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.50 s (backtrack: 0/20).

[ebuild  N     ] perl-core/CPAN-2.290.0-r4::gentoo  USE="-test" PERL_FEATURES="ithreads (-debug) -quadmath" 870 KiB
[blocks B      ] <perl-core/CPAN-2.380.0 ("<perl-core/CPAN-2.380.0" is soft blocking virtual/perl-CPAN-2.380.0)

Total: 1 package (1 new), Size of downloads: 870 KiB
Conflict: 1 block (1 unsatisfied)

 * Error: The above package list contains packages which cannot be
 * installed at the same time on the same system.

  (perl-core/CPAN-2.290.0-r4:0/0::gentoo, ebuild scheduled for merge) pulled in by
    =perl-core/CPAN-2.290.0-r4

  (virtual/perl-CPAN-2.380.0:0/0::gentoo, installed) pulled in by
    =virtual/perl-CPAN-2.380.0 required by (dev-lang/perl-5.42.0-r1:0/5.42::gentoo, installed) USE="gdbm -berkdb -doc -minimal" PERL_FEATURES="ithreads (-debug) -quadmath"
    >=virtual/perl-CPAN-2.290.0 required by (dev-lang/perl-5.42.0-r1:0/5.42::gentoo, installed) USE="gdbm -berkdb -doc -minimal" PERL_FEATURES="ithreads (-debug) -quadmath"


For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

https://wiki.gentoo.org/wiki/Handbook:X86/Working/Portage#Blocked_packages