Perhaps consider approaching the question from another angle and simply have two template vars.

This uses HTML::Template to demonstrate a possible way to do it.

#!/usr/local/bin/perl

use strict;
use warnings;

use HTML::Template;

my $tmpl = do{local $/;<DATA>};

my $t = HTML::Template->new(scalarref => \$tmpl);

$t->param(
  vehicle_uc => q{BUS},
  vehicle_lc => q{bus},
);

print $t->output;

__DATA__
sam is riding an uppercase <TMPL_VAR vehicle_uc>
sam is riding an lowercase <TMPL_VAR vehicle_lc>
[download]

sam is riding an uppercase BUS
sam is riding an lowercase bus
[download]