promise(\ my $result_of_long_compute, sub {
  # Long computation here
});

##</code><code>##

sub promise {
  my ($to_tie, $implement, $opts);
  if (UNIVERSAL::isa($to_tie, 'SCALAR')) {
    tie $$to_tie, Data::Lazy, $implement, $opts;
  }
  elsif (UNIVERSAL::isa($to_tie, 'HASH')) {
    ...
}