If you want to do it one-off, then you can simply set $! before you call die. If you want to set it more globally, then you might try:

  $SIG{__DIE__} = sub { $! = 42; die @_ };
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If you ever catch errors using eval, then you might want to be a bit more careful:

  $SIG{__DIE__} = sub { $! = 42 unless $^S; die @_ };
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