This is one of 4 packages which are still in my local overlay and which seem to be forgotten by the recent "perl-5.20" conversion: corelist sugggests that perl-5.18 contains virtual/perl-IO-Compress-2.60.0, but none of the ~virtual/perl-IO-Compress-2.60.0 ebuilds is satisfied if perl-5.18 is installed. Moreover, perl-5.20 seems to contain even virtual/perl-IO-Compress-2.64.0 for which there is not even a corresponding perl-core version in the tree. If you do not want to add this new version to the tree, I suggest that you let virtual/perl-IO-Compress-2.60.0 be also satisfied by perl-5.20, since it seems rather counterproductive to force users of perl-5.20 to install an older version of that package.
virtual/perl-IO-Compress-2.60.0-r1 is satisfied by Perl 5.18 virtual/perl-IO-Compress-2.64.0 (masked) is satisfied by Perl 5.20