$foo = reverse $foo;
$foo =~ s/b/X/;
$foo = reverse $foo;
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there's probably a more succinct way to write this.

Update: i haven't tested this (or the other one actually), but something like this might work... find a 'b' that isn't followed by any more 'b's:

s/b[^b]*$/X/;
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Update:
After reading Transient's post (below), i realized my second regex is wrong. should be s/b([^b]*)$/X$1/, but now i feel like i just ripped it off from him.