sub showStart {
  my $self = shift;
  # function specific code ...
  return $self->display_tmpl(
    file => 'start.phtml',
    # other parms
  );
}

sub display_tmpl {
  my $self = shift;
  my %opts = shift;
  my $tmpl = $self->load_tmpl('main.phtml');
  my $tmpl_sub = $self->load_tmpl($opts{file});
  #... use other parms...
  $tmpl->param(tmpl_sub => $tmpl_sub->output());
  return $tmpl->output();
}