Dragonchild (and all who followed) - thank you.
This line did the trick for me:
perl -pe "s/([^?])'/$1'/g" input.txt
Things you folks taught me:
grouping in a regex - see perlrequick
the special variable $1 - ditto
the -p switch
the -i switch
check code before posting. I had two errors in mine (extra double quote and unnecessary escape)
Cheers - Winter