$s='"crosby","stills","nash",""and" ""sometimes"" "young""'

# capture everything between a quote and a quote 
# follow by a comma or the end of string
 $s =~ s[ " (.*?) " ( , | $ ) ]
 { 
 	# Look for unescaped embedded quotes and escape them
 	(my $t = $1) =~ s[ (?<!") " (?!") ][""]gx; 
 	
 	# put back the outer quotes and the comma if there was one
 	'"' . $t . '"' . ($2||''); 
 }gex;

print $s;
"crosby","stills","nash","""and"" ""sometimes"" ""young"""