feat(perl): add perlbrew auto activation

[jamesrtnz]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Add code to perl plugin to activate perlbrew if 'perlbrew' function not defined already

Other comments:

Currently, the PERL plugin defines some aliases for perlbrew, but doesn't check or activate perlbrew. This PR adds (optional) perlbrew activation to the plugin. It will check if the 'perlbrew' function is defined; and if not, attempt to source the activation code included in perlbrew. The perlbrew root directory can be defined by the standard PERLBREW_ROOT variable; and then entire activation can be disabled by setting PERLBREW_NO_AUTO_ADD to some value before sourcing oh-my-zsh.

[carlosala]

LGTM 🚀

[carlosala]

Thanks for your contribution! I refactored a bit the code and documented before merging 😊