dd() is a function exported by Data::Dump. Since %hash as argument to dd would be expanded into a flat list, a reference to %hash is passed to dd. That's what the backslash does, it is the reference operator. See perlref.
The next lines are sample output produced by dd.
perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
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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"],
}
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.
Give a man a fish: <%-{-{-{-<
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this is not working ...
"This does not work" is almost useless as a problem description. What does not work? Where is your code? Why can you not provide your code and short examples of an input file and expected output file(s)? Please, please, please see Short, Self-Contained, Correct Example for further discussion of why these steps are vital for getting help.
Give a man a fish: <%-{-{-{-<
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