[perl #6006] Regexp replaces using large replacement variables fail some of

Actually the patch wasn't good, since it marked as non-constant too many
types of substitutions.
So I reverted it and committed another fix instead:

Change 29617 on 2006/12/23 by rgs@marais

        A better fix for RT #6006: revert change 29616, which was bogus,
        and add OP_SCOPE to the list of "dangerous" ops that might appear
        on the right side of s///.
        However, this fix does not work under taint mode, and op/subst.t
        has -T.

I added a corresponding TODO test (this bug is still present with -T).

• [ID 20010311.001] Regexp replaces using large replacementvariables fail some of the time by ian
• [perl #6006] Regexp replaces using large replacement variables fail some of by Rafael Garcia-Suarez via RT
• [perl #6006] Regexp replaces using large replacement variables fail some of by Rafael Garcia-Suarez via RT
• [perl #6006] Regexp replaces using large replacement variables fail some of by Rafael Garcia-Suarez via RT
• [perl #6006] Regexp replaces using large replacement variables fail some of by yves orton via RT
• Re: [ID 20010311.001] Regexp replaces using large replacement variables fail some of the time by Hugo
• Re: [ID 20010311.001] Regexp replaces using large replacement variables fail some of the time by Simon Cozens