As is pretty much always the case with parsing questions, B::Deparse is your friend.

$ perl -MO=Deparse,-p,-q -we '$File::Find::prune= not $recurse and ($F
+ile::Find::dir ne $File::Find::topdir);'
Useless use of string ne in void context at -e line 1.
Name "File::Find::prune" used only once: possible typo at -e line 1.
Name "File::Find::topdir" used only once: possible typo at -e line 1.
Name "File::Find::dir" used only once: possible typo at -e line 1.
Name "main::recurse" used only once: possible typo at -e line 1.
BEGIN { $^W = 1; }
(($File::Find::prune = (!$recurse)) and ($File::Find::dir ne $File::Fi
+nd::topdir));
-e syntax OK
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and has a lower precedence than =; I think in fact someone posted a meditation on just this problem not last week.

Update: Meh, it was two weeks ago: Burned by precedence rules.