And if you'd rather use CGI::Ex::Template (which is Template::Toolkit compatible template wise) you could use the following code (I'd use CGI::Ex::Template personally - but that might be because I'm intimately familiar with it :)).

use CGI::Ex::Template;

my $cet = CGI::Ex::Template->new(
    UNDEFINED_GET => sub {
        my $self = shift;
        my $ident = shift;

        print "(".join("|",@$ident).")\n";

        if (@$ident > 2) {
            return '';
        } elsif ($ident->[0] eq 'adjective') {
            return 'red';
        } elsif ($ident->[0] eq 'noun') {
            return 'fox';
        } elsif ($ident->[0] eq 'verb') {
            return 'jumped';
        } else {
            return '';
        }
    },
);

my $string = "The [% adjective %] [% noun %] [% verb %] over the river
+.\n";
$cet->process(\$string);
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