def main((blah=s 'default_prefix')) : template {
  prepend_rofl_arg [% IF blah %] --rofl $blah [% END %] | dont_prepend_lol_arg --lol BREAKAGE
}

def prepend_rofl_arg(rofl=s) : perl -n : env {
  print "$ENV{rofl}$_";
}

# verifies env var doesn't get installed unless you set :env
def dont_prepend_lol_arg(lol=s) : perl -n {
  print "$ENV{rofl}$_";
}