Hi,

I could make it work with :

perl -pe 's/^([^"]+)"(.+)"/($x = $2) =~ y{\t}{,}s;"$1\"$x\""/e'
[download]

Here is another try for the input file :

a    b    c d    "x1    x2"
c    d    e    "x2"
e    f    h    "x3    x4 x5    x6"
[download]

The expected result :

a    b    c d    "x1,x2"
c    d    e    "x2"
e    f    h    "x3,x4 x5,x6"
[download]

Thanks again to all of you for your useful help.
Regards.
Xuo.