Here's mine, a different approach, based on the idea "replace what you want to keep by itself":

s/(,)|(".*?")/ $1 || $2 =~ s(,)( )gr /ge;
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This requires a "fairly recent" perl, though I'm unsure exactly when s///r was introduced.

For older (and for newer) perls, you can use the slightly more verbose:

s/(,)|(".*?")/ $1 || do { (my $s = $2) =~ s(,)( )g; $s } /ge;
[download]