Global scoping question

I'm having a strange global scoping situation.

I've defined an action hook function that's called from my plug-in. I've verified the hook is getting called as is the action hook function itself. But although I've specified some WP globals inside that action function (like $pages, $post, etc -- along with some basic string globals) with the global statement, when I modify the globals inside my function, they don't change in the WP/PHP global scope.

However, if I use the GLOBALS super-global, and I do something like set $GLOBALS['foo'] = bar, then the global gets updated in the global scope.

I'm assuming I'm doing something wrong because I've never seen other actions use lots of super-globals, but I can't seem to get the settings inside the function to go outside. And, just to make sure it's clear, I am declaring the variables that should be global as global inside my action function.

Any advice would be much appreciated.