2.html.ep

%= include 'header';
<%= $callee %> <%= $name %>!
Some text here

mojo-template-with-include.pl

#!/usr/bin/perl

# skipped

use strict;
use utf8;
use open qw( :std :utf8 );

# skipped

use Mojolicious;
my $template = '2.html.ep';

# skipped

my $mt = (my $app = Mojolicious->new)->renderer;

# skipped
my $callee = 'Dear';
my $first_and_middle_name = 'John';

my $out = $mt->render($template, {
    'callee' => $callee,
    'name'   => $first_and_middle_name,
});
print $out;
# Can't locate object method "stash" via package "2.html.ep" (perhaps you forgot to load "2.html.ep"?) at /usr/share/perl5/Mojolicious/Renderer.pm line 78.