XML::Simple has been regarded as the easier of the two.

XML::Simple is the hardest XML parser to use.

$smart->{book}('@')
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is

ref($simple->{book}) && ref($simple->{book}) eq 'ARRAY'
   ? $simple->{book}
   : [ $simple->{book} ]
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or

ForceArray => [qw( book )]  # In constructor
$simple->{book}
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XML::Smart appears to be an attempt to fix this problem of XML::Simple. (It still suffers from other problems of XML::Simple.)