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".