Roy Johnson has a working solution. The following will also do the trick:

perl -le '$_="filename.ext"; s/^(.*)\.([^.]*)$/$2:$1/; print($_);'
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Both Roy's and mine splits at the last ".", just like you were trying to do.

By the way, I would further postulate that extentions don't have spaces in them:

perl -le '$_="filename.ext"; s/^(.*)\.([^.\s]*)$/$2:$1/; print($_);'
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