Subversion --Perl

Patch suggested by @pwagland that should finally fix the --perl binding mv issue that causes the entire formula to error out if --perl build option is invoked. The issue & consequent patch will not affect anyone who doesn't build subversion with perl.

I've included a # explanation as to why the change has been made, as hopefully this patch can be replaced either by a subversion fix or a better Ruby-invoked fix in time. Although this could work as a permanent fix, the hope is that it's a temporary workaround until something cleaner is worked out.

See the following issues for more detail:

[1](https://github.com/Homebrew/homebrew/issues/30370)
[2](https://github.com/Homebrew/homebrew/issues/30224)
@mikemcquaid Your fix didn't seem to do the job for me either. @pwagland's edit did, and fits the style you requested, so I'll submit a new commit with that and hopefully that'll be okay with you.