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When I add use warnings; use strict;, there are warnings which give you a clue as to why it might be failing:

use strict;
use warnings;
use Data::Dumper;
my %hash;
open(FILEHANDLE, "<hash.dat") or die("boo! error");
while(<FILEHANDLE>) {
#    print $_;
    %hash = split(" ", <FILEHANDLE>);
}
print Dumper(\%hash);

__END__
Odd number of elements in hash assignment at .., <FILEHANDLE> line 2.
$VAR1 = {};
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