$ perldoc -f ==
No documentation for perl function '==' found
$ sudo apt-get install perl-doc
[sudo] password for bob: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
perl-doc is already the newest version (5.26.1-6ubuntu0.2).
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
$