Okay, there's something like this already there. For values that should be accessible anywhere during the life of the current request, you can use $r->pnotes(). It's just like a hash:

my $r = Apache->request(); # or grab it when passed to handler
$r->pnotes( Login => 1 );
my $foo = $r->pnotes( 'foo' );
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The pnotes() values only last for the length of the current request and get cleaned up automatically at the end of it. I'm not quite sure what you were trying to do with your provate_variable call, so I can't suggest anything there.