Question : How can I erase or clear the hash before my perl script takes the second file ?

The simplest way is to declare it in such a way that it goes out of scope when you stop processing the file. Something along the lines of:

for my $filename (@files) {
    my %data1;
    # do all processing of file $filename here
}
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Alternatively you can use undef %data1

my %data1 = %$list_a_ref; # Dereference lists

That doesn't just dereference, it also creates a copy. Do you want that?