dd \%hash;

This statement "dumps" data from a data structure. The data structure must be passed to  dd as a reference, hence  \%hash takes a reference to the hash. See Data::Dump::dd(). (Note: There are many data dumpers. I like Data::Dump, but it's not core. The Data::Dumper module comes as a part of the core Perl "standard" installation.)

{
  bar => ["13 : //comment thirteen"],
  foo => ["12 : //comment twelve ", "5  : //comment five"],
}
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This is the output of the  dd \%hash; statement. It appears immediately after the
    c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le " ... "
command line invocation used to execute the example code.