$_ = q{
xxx "xxxxx xxx xxxx" 'xxxx xx xx' xxxx xxxx "xxxxx " "xxx" xxx
};

s/
   \G
   ( # $1
      (?:
         (?:"(?:[^"\\]|\\.)*")  # Skip double-quoted phrases.
         |
         (?:'(?:[^'\\]|\\.)*')  # Skip single-quoted phrases.
         |
         \s                     # Skip whitespace.
      )*
   )
   ( # $2
      [^\s"']+                  # Something to prefix.
   )
/$1A$2/gsx;

print;


__END__
output:
======
Axxx "xxxxx xxx xxxx" 'xxxx xx xx' Axxxx Axxxx "xxxxx " "xxx" Axxx
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