IMO, when you need to "grep" stuff, you're better off doing it in perl. I'm agnostic about shell "du" vs. perl-module solutions for the same thing, but stringing a bunch of separate "grep" commands in a pipe like the OP does is just ugly and so unnecessary. I'd be content with something like this (not tested):

$du = join( '',
            grep !/^.*?Maildir.*?(?:cur|tmp|new|courierimapkeywords)$/
+,
            `du -ch $homedir` );

print $du;
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Anything you can do to shorten/constrain/simplify the stuff in backticks is generally a good idea, esp. in terms of eliminating "shell-magic" characters as much as (or completely if) possible.