$ sudo apt-get update
$ sudo apt-get install libhash-flatten-perl 

libhash-flatten-perl is installed in your system.

Make ensure the libhash-flatten-perl package were installed using the commands given below,

$ sudo dpkg-query -l | grep libhash-flatten-perl *

You will get with libhash-flatten-perl package name, version, architecture and description in a table

libhash-flatten-perl package basic information: