To get a backreference to a quote, you have to put the quote in parens, which means it is going to be returned as a separate group. So I think you're going to have to stay with the separate alternatives for each type of quote.

The /x option is absolutely straightforward: any whitespace within your regex is ignored. So you can pretty it up as you like. You can also put comments in it. I recommend you jump right into using it.

The \G anchor tells the pattern to resume looking from where it last left off with the string. I don't think it's going to help you with what you're trying to do here.

I notice that the backslash-protection of quotes doesn't work with your pattern. Consider that, within quotes, you will accept backslash followed by any character, and any run of non-backslash, non-quote characters. Or, you will accept a minimal match of any character leading up to a quote that is not preceded by a backslash. I illustrate both of these here (along with the use of /x):

my @matches = m/([^"']+
                |(?: " (?:\\.|[^\\"]+)* " ) # Double quote
                |(?: ' .*? (?<!\\)' ))/gx;
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Update: note that the second version will not recognize that \\' does not protect the quote.