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

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