I agree with ikegami that the lack of symetry feels weird, even though its perfectly valid code.

Personally, I'd take the second approach but specificially limit the scope of $id (by use if the outer {}s).

{ ### Do this or that for $obj
  my $id;

  if ( $id = $obj->get_id() ) {
    $obj->do_this();
  } elsif ( $id = $obj->create_id() ) {
    $obj->do_that();
  } else {
    $obj->do_somethingelse();
  };
};

{ ### Do this and that for $next_obj
  my $id;

  if ( $id = $next_obj->get_id() ) {
    $next_obj->do_this();
  } elsif ( $id = $next_obj->create_id() ) {
    $next_obj->do_that();
  } else {
    $next_obj->do_somethingelse();
  };
};
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