unixbench | repository contains the setup and runtime configurations

The OpenEmbedded build system aggressively splits default Perl package into tiny modules minimizing size of the target system (when you don't need all core Perl modules you just don't install all of them), so to install 'perl' package there is not the same as to install 'perl' package in Debian or some other standard distro. You can see what is installed with 'perl' via opkg files perl command (if you're using ipk packages, of course).

So what you need is to install additional packages that are already built when building Perl. They have very simple naming scheme of perl-module-*, so for your package that would be perl-module-posix, perl-module-time-hires, perl-module-io-handle and perl-module-findbin. Of course they can also have some dependencies, so better make a proper recipe for your program and specify RDEPENDS appropriately.