One key thing that I think you are doing incorrectly is you are clearing the contents of @rangelist after you store a reference to it in the hash. In the end, each hash value will be a reference pointing to the exact same array, which will probably be empty.

@rangelist = (); #just keeps dumping the data stored there.
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Change @rangelist into $rangelist and make it a reference to an anonymous array. Then when you need another array, you just declare another one and assign it to $rangelist.

Like so:

unless($_ =~ /^\d+\.\d+\.\d+\.\d+/) {
   unless ($key eq "") {
       $CoreRegR{$key} = $rangelist;
   }
   $rangelist = [];
   $key = $_;
} else {
   push(@$rangelist,$_);
}
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HTH

P.S. Check out the standard Data::Dumper module to print out complex data structures so you can more easily see what is being built.