In template toolkit:

my $lazy = do {
  my $result;
  sub {
    unless (defined $result) {
      ... do expensive computation ...
      ... assigning to $result ...
    }
    return $result;
  };
};
my %vars = {
  lazy => $lazy,
  easy => 2 + 2,
};
$template->process('mything.tt', \%vars);
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From the template, you use both "lazy" and "easy" the same way. Except that "lazy" is a coderef. That's what's cool about TT: the call is the same.

I seem to recall a module that does this for you. Yes, Data::Lazy.