# emerge -av1 dev-lang/perl

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

Calculating dependencies... done!

!!! All ebuilds that could satisfy "dev-lang/perl" have been masked.
!!! One of the following masked packages is required to complete your request:
- dev-lang/perl-5.12.3::gentoo (masked by: invalid: DEPEND: USE flag 'elibc_FreeBSD' referenced in conditional 'elibc_FreeBSD?' is not in IUSE)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.