c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $string (
   '\"my\" \"dog\"',
   '\"my\" \"dog shepherd\"',
   'my dog',
   'my \"dog shepherd\"',
   '\"my \"dog\"',
   'my \"dog shepherd',
   ) {
   my $paired   = qr{ \x22 [^\x22]* (?: \\. [^\x22]*)* \x22 }xms;
   my $unpaired = qr{ \x22 [^\x22]* \z }xms;
   ;;
   my $orphan = $string =~ m{ $paired (*SKIP) (*FAIL) | $unpaired }xms;
   print qq{'$string' }, $orphan ? 'INVALID' : 'valid';
   }
"
'"my" "dog"' valid
'"my" "dog shepherd"' valid
'my dog' valid
'my "dog shepherd"' valid
'"my "dog"' INVALID
'my "dog shepherd' INVALID