use warnings;
use strict;
use Template;

my $tt = Template->new();
my %data_href = (
                 fname => 'William',
                 lname => 'Shakespeare',
                 );

$tt->process( \*DATA, { data_href => \%data_href, } )
    or die $tt->error(), "\n";

__END__
My original template
====================
First name : [% data_href.fname %]
Last name  : [% data_href.lname %]

##</code><code>##

My original template
====================
First name : William
Last name  : Shakespeare

##</code><code>##

use warnings;
use strict;
use Template;

my $tt = Template->new();
my %data_href = (
                 fname => 'William',
                 lname => 'Shakespeare',
                 );

my %data_map = (
                first_name => 'fname',
                last_name => 'lname',
                );

$tt->process(
             \*DATA,
             {
                 data_href => {
                     map { $_ => $data_href{ $data_map{$_} } }
                     keys %data_map
                 },
             }
             )
    or die $tt->error(), "\n";

__END__
New third party template
========================
First name : [% data_href.first_name %]
Last name  : [% data_href.last_name %]

##</code><code>##

New third party template
========================
First name : William
Last name  : Shakespeare

##</code><code>##

use warnings;
use strict;
use Template;

my $tt = Template->new();
my %data_href = (
                 fname => 'William',
                 lname => 'Shakespeare',
                 );

my %data_map = (
                first_name => 'fname',
                last_name => 'lname',
                );

$tt->process(
             \*DATA,
             {
                 map { $_ => $data_href{ $data_map{$_} } }
                 keys %data_map
             },
             )
    or die $tt->error(), "\n";

__END__
New third party template with individual varnames
=================================================
First name : [% first_name %]
Last name  : [% last_name %]