Maybe DDS or something used by it installs (and later removes) a DIE handler, leaving the key but undefing the value?

Update:Seems to be the case:

c:\test>perl -MData::Dump::Streamer -MData::Dumper -E"say Dumper \%SIG
+"
$VAR1 = {
          'ABRT' => undef,
          'BREAK' => undef,
          'HUP' => undef,
          'NUM19' => undef,
          'FPE' => undef,
          'CLD' => undef,
          'NUM12' => undef,
          'NUM16' => undef,
          '__DIE__' => undef,
          'NUM18' => undef,
          'NUM17' => undef,
          'STOP' => undef,
          'NUM24' => undef,
          'NUM10' => undef,
          'INT' => undef,
          'SEGV' => undef,
          'QUIT' => undef,
          'KILL' => undef,
          'PIPE' => undef,
          'NUM05' => undef,
          'CONT' => undef,
          '__WARN__' => undef,
          'NUM06' => undef,
          'NUM07' => undef,
          'ILL' => undef,
          'TERM' => undef,
          'ALRM' => undef,
          'CHLD' => undef
        };
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