what you are looking for is called "negative look ahead"

see http://perldoc.perl.org/perlretut.html#Looking-ahead-and-looking-behind

  DB<100> $txt=q( This is "an example", but "`this not"'. )
 => " This is \"an example\", but \"`this not\"'. "
 
  DB<101> $txt =~ s/"(?![`'])/'/g
 => 2
 
  DB<102> $txt
 => " This is 'an example', but \"`this not\"'. "
[download]

but you might get bitten by shell escaping if you try a one-liner