I tried to make changes you proposed, but, it didn't work: instead of

s/throw "[^"]+"/throw ($1)/
[download]

The result semms like using:

s/throw "[^"]+"/throw (\$1)/
[download]

So no substitution using matched values is performed, I guess I need that substitution regexp operator would evaluate the $substitute, whereas now it doesn't and just replaces using literal value. P.S. Anarion's suggestion also didn't work for me:

C:\Projects\Config2\9XImage.cpp:
           throw "Error writing to floppy!";
    qq<throw ("$1")>
[download]

Also literal evaluation.