ip-10-226-66-16 ~ # cat /etc/portage/package.mask
>=dev-db/mysql-5.1.59
>=virtual/mysql-5.1
>=virtual/mysql-5.5
>=dev-perl/DBD-mysql-4.20.0
>=dev-perl/DBD-mysql-4.01.7
ip-10-226-66-16 ~ # cat /etc/portage/package.accept_keywords
sys-kernel/xen-sources
ip-10-226-66-16 ~ # cat /etc/portage/package.keywords
#>=virtual/mysql-5.5 ~amd64
>=dev-db/mysql-5.5.19 ~amd64
ip-10-226-66-16 ~ # emerge -pav =dev-db/mysql-5.5.19

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

Calculating dependencies... done!
[ebuild  N     ] dev-db/mysql-init-scripts-2.0_pre1-r2  0 kB
[ebuild  N     ] dev-perl/Net-Daemon-0.43  28 kB
[ebuild  N     ] perl-core/Storable-2.20  174 kB
[ebuild  N     ] perl-core/File-Spec-3.31  132 kB
[ebuild  N     ] perl-core/Sys-Syslog-0.290.0  78 kB
[ebuild  N     ] app-arch/libarchive-2.8.4-r1  USE="acl bzip2 lzma zlib -expat -static -static-libs -xattr" 1,468 kB
[ebuild  N     ] virtual/perl-Storable-2.20  0 kB
[ebuild  N     ] virtual/perl-File-Spec-3.31  0 kB
[ebuild  N     ] virtual/perl-Sys-Syslog-0.290.0  0 kB
[ebuild  N     ] dev-util/cmake-2.8.6-r4  USE="ncurses -emacs -qt4 -test -vim-syntax" 5,454 kB
[ebuild  N     ] dev-perl/PlRPC-0.202.0  18 kB
[ebuild  N     ] dev-perl/DBI-1.615  USE="-test" 558 kB
[ebuild  N    #] dev-db/mysql-5.5.19  USE="berkdb community perl ssl -cluster -debug -embedded -extraengine -latin1 -max-idx-128 -minimal -profiling (-selinux) -static -systemtap -test" 25,411 kB
[ebuild  N    #] virtual/mysql-5.5  USE="-embedded -minimal -static" 0 kB
[ebuild  N    #] dev-perl/DBD-mysql-4.20.0  USE="-embedded" 135 kB

Total: 15 packages (15 new), Size of downloads: 33,453 kB

The following keyword changes are necessary to proceed:
#required by dev-db/mysql-5.5.19[perl], required by virtual/mysql-5.5
>=dev-perl/DBD-mysql-4.20.0 ~amd64
#required by dev-db/mysql-5.5.19, required by =dev-db/mysql-5.5.19 (argument)
>=virtual/mysql-5.5 ~amd64

The following mask changes are necessary to proceed:
#required by dev-db/mysql-5.5.19[perl], required by virtual/mysql-5.5
# /etc/portage/package.mask:
>=dev-perl/DBD-mysql-4.20.0
#required by dev-db/mysql-5.5.19, required by =dev-db/mysql-5.5.19 (argument)
# /etc/portage/package.mask:
>=virtual/mysql-5.5
#required by virtual/mysql-5.5, required by dev-perl/DBD-mysql-4.20.0
# /etc/portage/package.mask:
>=dev-db/mysql-5.5.19

NOTE: This --autounmask behavior can be disabled by setting
      EMERGE_DEFAULT_OPTS="--autounmask=n" in make.conf.
ip-10-226-66-16 ~ #