Yeah, I know I'm confused!

If @ARGV is empty, STDIN is used to supply the contents, not to supply the list of filename(s)

That's a good explanation!
I think where I went wrong was initially experimenting with the -n and -p switches in the form

find ... | perl -ne 'do something'
[download]

and then when I added -i I though it would automatically edit the files the same way

find ... | perl -nie 'do something'
[download]

and I didn't know why I was having to use

find ... | xargs perl -nie 'do something'
[download]

to get my desired results.
Thanks for your help.