crazyinsomniac pointed me at japhy's book, which shows it's possible. For awhile it then seemed it could only be done with $_. (Though one could eschew the issue with @+.) Zaxo and crazyinsomniac then proceeded to set me straight. I'm not sure what was wrong when I played around with it earlier... But then again I have had odd things happen today such as forms on a page fill-in values and submit themselves...

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perl -pe "s/\b;([st])/'\1/mg"