perlfunc: update require() pseudocode

RT #120292

The entry for require() in perlfunc has a perl function that's supposed to
illustrate how require works is rather out of date. In particular, the
fact that the search stops on EACCES wasn't incorporated, and the various
ways of it failing and reporting errors was inaccurate.

I also removed the explicit code for executing the 'do' in the caller's
package, and replaced it with just the comment 'but run in caller's
namespace', since the details were a bit obscure.

Finally, I re-indented it since it had strange 3-space indents