I see no reason in your code to delay execution of show_elements. You can just change that sub to return a list of stuff, instead of having it output HTML items directly (which is the whole point of using templates in the fist place):

sub show_elements ($)
{
    my $num = $_[0];
    return [] unless( $num > 0 );
    return [ 1..$num ];
}
...
my $data = {
             show_elements => show_elements( $elements ),
           };

then in the template:

<ul>
{# FOREACH e = show_elements #}
  <li>{# e #}</li>
{# END #}
</ul>

If you really need to delay the sub call, have a look at the TT docs which explain it quite adequately.

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