This structure is a HashRef that contains ArrayRef. When you see { } its a HashRef, when you see [ ] its an ArrayRef. To loop through them, you have to dereference them.

Lets say your reference variable is called $VAR1 (obviously this name comes from Data::Dumper so use the real variable). You can do this:

# Bellow, you have two dereference
# The arrow dereferences the HashRef $VAR1
# The @{ } dereferences the ArrayRef under 'book' =>
foreach my $book ( @{ $VAR1->{'book'} } ) {

  # Then, each $book is another HashRef
  print $book->{'isbn'}, "\n";
  print $book->{'title'}, "\n";

  # But 'author' is an ArrayRef:
  foreach my $author ( @{ $book->{'author'} } ) {
    
    print "Author: ", $author, "\n";
  }
}
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That does not print the XML code, of course, but nothing prevents you from reprinting the labels manually. Otherwise, you have to use an XML related module.