Test::Builder: Fix "return $a or $b" precedence issue

perl parses "return $a or $b" effectively as "return $a;", which is
not what was intended.

Signed-off-by: Niels Thykier <niels@thykier.net>
op.c: Warn on "return $a or $b" [perl #59802]

Add a warning for the (likely) unintended use of "return $a or $b"
(and similar expressions), which perl parses as "(return $a) || $b"
(which is effectively just "return $a;").

Note this warning is triggered by some modules (e.g. Test::Builder).
These are not fixed by this commit.

Signed-off-by: Niels Thykier <niels@thykier.net>
Use dev version for Test::Builder et al.

It has changes in blead that are not on CPAN yet.  The number
0.98_05 is already used on CPAN.
Fix precedence issue in JSON::PP
Use dev version for JSON::PP
op.h: Describe entersub and rv2cv flags

Entersub ops can turn into rv2cv ops at compile time.  It is important
that possible flag conflicts are handled properly.  Since it took me
a while to figure out that there were no bugs here, here are my own
notes, cleaned up and nicely formatted.