%hash = undef; produces a hash with a single (weird) key

The key gets upgraded to a string, so it is really the null string:

use warnings;
use strict;

my %hash = undef;
use Data::Dumper;print Dumper \%hash;
__END__
Odd number of elements in hash assignment at - line 1.
Use of uninitialized value in list assignment at - line 1.
$VAR1 = {
          '' => undef
        };
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