No cause for pause. undef is promoted to the empty string, used as a hash key and assigned undef as its value because that's the default value of items in the empty list | used by Perl when a key in a hash assignment list has no corresponding value, or Odd number of elements in hash assignment.

Win8 Strawberry 5.8.9.5 (32)  Sat 08/07/2021 23:15:45
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Data::Dump qw(dd);

my %h = undef;
print scalar %h, "\n";

dd \%h;

^Z
Odd number of elements in hash assignment at - line 7.
Use of uninitialized value in list assignment at - line 7.
1/8
{ "" => undef }
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Update: Clarified (I hope!) the explanation of odd-item hash assignment.