Well it still seems strange that when the rest of the whole song DOES rhyme, there's this one line that DOESN'T in the most commonly sung version. I'll stick with "perfuming" if I'm doing it solo (but then you never are as everyone else expects "grows around the blooming heather")!
A friend just wrote a new song, sang it through to me: after she'd finished, said, "Did you notice it wan't it rhyme?" To be honest, I hadn't, tho' the words were excellent and really told a story! But I'd say, be consistent: either rhyme all the way through, or don't rhyme at all.
And I guess "Santa" has blocked (blacked?)sinuses from climbing dowm all those chimneys and can't smell the heather perfuming the moorlands?