In my own code i always local'ize $_ using for, as recommended in Camel III:

Solution 2

sub dequote {
   my $string = @_ ? $_[0] : $_;
   for ($string) {
       s/^"//;
       s/"$//;
       s/\\(.)/$1/g;
       return $_;
   }
}
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It's IMO even more readable than the local $_ variant, and passes all your tests.