rxp =~ str or str =~ rxp ?

Is there a preferable order, when using the match operator "=~" with
regex/string?

I notice that  the pickaxe book uses one prder in the text, and another in
the library reference.

# from Standard Types in pickaxe
a = "Fats Waller"
a =~ /a/ 1
a =~ /z/ nil
a =~ "ll"

# from libray reference in pickaxe
/SIT/  =~ "insensitive" nil
/SIT/i =~ "insensitive" 5

(1) The first use (str=~rxp) seems more koherent with the string/string use.
I might not see the whole picture though. Is there any consensus in the use?
Or opinions on preferable use?

(2) I know that both ways work just fine in the current implementation, but
to mix them in the same program seem not to add clarity.

(3) The match operator IMHO seems to somewhat resemble the set
inclusion/"is-a-subset-of" operator of mathemathics, the 90 degrees turned
"U". Then it would seem logical that it does imply a direction.

Thankful for any directions!

amike via Henning