# file test.pl

use HTML::Template;
use strict;
use warnings;

my $template = HTML::Template->new(filename => 'test.tmpl');

# params
$template->param(tag1_common => 'XXX');
$template->param(tag2_common => 'YYY');
$template->param(tag3_common => 'ZZZ');

print $template->output();

# file test.tmpl

<root>
    <tag1>
        <commontag><TMPL_VAR ESCAPE=HTML tag1_common></commontag>
    </tag1>
    <tag2>
        <commontag><TMPL_VAR ESCAPE=HTML tag2_common></commontag>
    </tag2>
    <tag3>
        <commontag><TMPL_VAR ESCAPE=HTML tag3_common></commontag>
    </tag3>
</root>

# output

<root>
    <tag1>
        <commontag>XXX</commontag>
    </tag1>
    <tag2>
        <commontag>YYY</commontag>
    </tag2>
    <tag3>
        <commontag>ZZZ</commontag>
    </tag3>
</root>