The template is populated in the constructor of a plugin, but you never call the constructor. Also, you probably don't want to create a hash with just the template in the constructor, you want to call SUPER::new and add template to it...

Update: BTW, /[data]/ matches a character class, not the literal string [data].

Update 2: The real problem is you're using inheritance, but Service is not a parent, it's a factory.

Update 3: Like this:

---./t/00.t---

#!/usr/bin/perl
use warnings;
use strict;

use Test::More;
use Service::Factory;

my $xml = Service::Factory->instantiate('xml', 'xmldata');
$xml->populate;
like $xml->{template}, qr{<data>xmldata</data>}, 'XML OK';

my $json = Service::Factory->instantiate('json', 'jsondata');
$json->populate;
like $json->{template}, qr/{"data":"jsondata"}/, 'JSON OK';

done_testing;
[download]

---./lib/Service/XML.pm---

package Service::XML;

use warnings;
use strict;

use parent 'Service';
use XML::LibXML;

sub new {
    my $class = shift;
    my $self = $class->SUPER::new(@_);
    my $xml = 'XML::LibXML::Document'->new;
    $xml->setDocumentElement($xml->createElement('data'));
    $xml->documentElement->appendTextNode('[data]');
    $self->{template} = $xml->toString;
    return $self
}

__PACKAGE__
[download]

---./lib/Service/Factory.pm---

package Service::Factory;

use Service::XML;
use Service::JSON;

use warnings;
use strict;

my %SUPPORTED = (
    xml  => 'XML',
    json => 'JSON');

sub instantiate {
    my ($class, $plugin) = @_;
    die "Unknown plugin '$plugin'.\n" unless $SUPPORTED{$plugin};
    my $subclass = "Service::$SUPPORTED{$plugin}";
    return $subclass->new(@_[2 .. $#_])
}
__PACKAGE__
[download]

---./lib/Service/JSON.pm---

package Service::JSON;

use warnings;
use strict;

use parent 'Service';

use Cpanel::JSON::XS qw{ encode_json };

sub new {
    my $class = shift;
    my $self = $class->SUPER::new(@_);
    $self->{template} = encode_json({data => '[data]'});
    return $self
}

__PACKAGE__
[download]

---./lib/Service.pm---

package Service;

use warnings;
use strict;

sub new {
    my ($class, $data) = @_;
    return bless {
        data => $data,
        service => $class,
    }, $class
}

sub populate {
    my $self = shift;
    $self->{template} =~ s/\[data\]/$self->{data}/i;
}

__PACKAGE__
[download]

---

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