As soon as undef is used as value, your code will break. Better be safe than sorry.

Debugger demo:

  DB<2> $h{a}=1

  DB<3> $h{b}=undef

  DB<4> x grep {defined $h{$_}} "a".."d"
0  'a'
  DB<5> x grep {exists $h{$_}} "a".."d"
0  'a'
1  'b'
  DB<6>
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