# 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>

##</code><code>##

# file test.pl

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

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

my @xml = (
    {
        parent          => 'tag1',
        common_value    => 'XXX',
    },
    {
        parent          => 'tag2',
        common_value    => 'YYY',
    },
    {
        parent          => 'tag3',
        common_value    => 'XXX',
    },
);

# params
$template->param( loop  => \@xml );

print $template->output();

# file test.tmpl

<root>
  <TMPL_LOOP loop>
    <<TMPL_VAR parent>>
        <commontag><TMPL_VAR ESCAPE=HTML common_value></commontag>
    </<TMPL_VAR parent>>
  </TMPL_LOOP>
</root>

# output

<root>

    <tag1>
        <commontag>XXX</commontag>
    </tag1>

    <tag2>
        <commontag>YYY</commontag>
    </tag2>

    <tag3>
        <commontag>XXX</commontag>
    </tag3>

</root>