When it's easier to write the shell script than the Perl version, for whatever value of "easier" passes your particular threshold. And it's going to be really, really subjective exactly what the threshold is from person to person and shell to shell.

Just as an example, where someone else might whip out File::Find (or File::Find::Rule) I might be just as likely to knock out a zsh one-liner right at my prompt since I've gotten fairly comfortable with it's pretty rich recursive globbing syntax. But then there's other stuff like moderately fancy CSV file manipulation that a better shell head might easily do with some arcane paste invocation that I'd break out Text::CSV_XS for.

If you're looking for generalities you'll likely get some pointers as to general areas of competency (like a shell's going to be better at chaining pipelines of commands together, where you'd be knee deep in system or open3 calls in Perl), but to know where it's going to be more appropriate for you you'll need to learn your tools and get comfortable enough that you know where your boundary lines are.