$str =~ s< 
           (           # Start capturing (to $1)
             \s*       # Optional whitespace (forgot this before)
             \"        # starting with a quote
             [^\"]+    # 1 more of anything except a quote
             \"        # another quote
             \s*       # optional white space
           )           # end capture
           (,|$)       # capture comma seperator (or EOL*)
         >
         <  # /e option makes this becomes a code block
            # put the contents of $1 into a localised writable var
            #($1 is read only!)
            # and use the tr/// op to  remove the commas
            (local $a = $1) =~ tr/,//d;  
            # The last expression in the block is used as the 
            # replacement expression, so we put the decomma'd localised var here.
            # not forgetting to put back the final comma
            # If it was there*
             $a .$2 ; 
         >gex; #!" Global, Execute, ignore whitespace in regex