Here's the platform I've been using for several years now, on every computer I work on. Note that it doesn't "split" code (ie. show only a single sub clicked on), but clicking on a sub within the editor will take you directly to it, and with code folding, you can see in your file only the sub definition lines until you unfold to view the code.

• intelliJ IDEA Full featured IDE
• Camelcade Perl5 Plugin Perl 5 support for the IDE (installable from within the IDE itself)
• Devel::Camelcadedb - Provides full Perl debugging support within the IDE

Here's an screenshot of the above setup. Normally I have two files open side-by-side within the IDE window, but when working on my laptop, the screen's just a tiny bit too small to do that efficiently.

Although I've only mentioned IDEA, the Perl 5 plugin works on all of intelliJ's IDEs (PyCharm, CLion, Rider etc), which is handy, because if I'm working on one of my XS distributions, I'm using the CLion IDE, but because the plugin is installed in this one as well, I don't have to switch back and forth between two IDEs to work on Perl or C/C++ code.

There are thousands of various plugins for the IDE in addition to the few I mentioned above, including editors (Vim, for the win!), and comes fully complete with VCS integration (I use Git exclusively nowadays). It also has a built-in terminal, and while the IDE is fully capable of running code, having the terminal built-in is fantastic, particularly because a lot of coding I do is over an SSHFS connection, so I can SSH to the remote system from right within the IDE itself.

Note that IDEs have a steep learning curve if all one is familiar with is a simple text editor, but it's worth it. A couple of good solid weeks of learning and using, and the time you'll save in the long run will be immense.

Update: I forgot to mention that the IDE mentioned also allows you to select any item, and it'll follow through to any sub or class in other any other file they're defined in (deep inspection). Exceptionally handy for delving deep.