Variation on the second approach you could load your values from an YAML file (or similarly JSON if that's your bag) rather than from code:

use Template;
use YAML::XS qw( Load LoadFile );
use Cpanel::JSON::XS qw( decode_json );

## This could be an external file, and/or make this a plugin argument 
+that points to it.
my $consts = Load( <<'EOT' );
---
PRE: 1
REGULAR: 2
POST: 3
EOT

## Alternately
my $alternate = decode_json( <<'EOT' );
{"PRE": 1, "REGULAR": 2, "POST": 3, "Foo": "Bar" }
EOT

my $template = Template->new({
  CONSTANTS => { y => $consts, j => $alternate },
  CONSTANTS_NAMESPACE => 'C',
});

__END__
[% IF type == C.j.REGULAR %]
Blah blah I R REGULAR stuff.
But JSON says foo is '[% c.j.Foo %]'
[% END %]
[download]