How do I do this without modifying File_a.pl?

I think you do want to modify File_a.pl to use parens instead of curlies List value constructors:

%test_hash = (a=>1, b=>2);
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Then you can require in File_b.pl:

require 'a.pl';

foreach (keys %test_hash){

print "$test_hash{$_}\n";

}
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