Hi. I'm not sure I understand what is so complex about your scenario. Perhaps you can provide more exact details about your input and output in your real-life scenario, and then we'd have some common ground to go on.

To get your output, you can easily modify the code I gave you:

# 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>
[download]

But perhaps what you're really asking about is the ability to have loops in your template. Well, if you would read the docs, you'd find you can do that too! Perhaps you're looking for something along these lines:

# 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>
[download]

Again, I'm just guessing here, since you're not providing enough detail about your actual needs.