FWIW, all 255 Perl Best Practices (without the text from the PBP book) are available here. There are a number of items related to heredocs in the Values and Expressions chapter, the most relevant being item 42:

Use a "theredoc" when a heredoc would compromise your indentation. This item offers two alternative ways to minimize compromising code indentation (which harms readability): i) factor out the heredoc into a predefined read-only constant at the file level; ii) factor out the heredoc inside a small subroutine (indentation is compromised but only in a small and isolated section of the code). Both methods do not indent the contents of the heredoc, so no error-prone tricks are required to adjust. Conway describes both methods as a "theredoc".

The book "Perl Best Practices"

Unfortunately my copy of that book is sat on a shelf in my office...I was last in my office back in February before the Pesky Pandemic turned the world upside down.

Come to think of it, I filled up the biscuit tin the last time I was there!

Speaking merely as a long time satisfied customer https://learning.oreilly.com/ (used to be called "Safari Books Online" back when I first subscribed) lets you get to electronic versions of all the ORA books as well as things from lots of other tech publishers as well as video courses. I've found it well worth what I pay them yearly that I don't spend on dead tree copies (then again I'm satisfied with reading things on my iPad).

Alternately keep an eye out as Humble Bundle periodically will have ORA pdfs in a collection (e.g. this current one; but nothing (directly) perl related in this one though).

The cake is a lie.
The cake is a lie.
The cake is a lie.