I tend to use:

sub new {
  my $class = shift;
  bless {
    _att1 => $_[0],
    ...
    _attN => undef,
  }, $class;
}
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I like to initialize a member attribute field in the blessed hash, even if it is not neccessarily defined during construction. I also like to minimize the number of input parameters in my constructors, but that really is just a personal preference.