$ perl -pe 's/^[^"]+"\s+(.+)\s+"/($x = $1) =~ y{ \t}{,}s; $x/e'
a b " x1 x2 "
x1,x2
c d " x3 "
x3
e f " x4 x5     x6 x7 "
x4,x5,x6,x7
x y z " 1space 2spaces  spacetabspace    end "
1space,2spaces,spacetabspace,end