I don't recommend that concept, actually. HTML::Template works great with DBI's selectall_arrayref. For example, something like this untested alteration of your code:

my $sth = $self->dbh->selectall_arrayref("
    SELECT rtime, dev, value
    FROM db_1361_asc
    WHERE device_id = ?
    AND rtime > ?
    AND rtime < ?
    ORDER BY rtime,dev
", {Slice =>{}} $device_id, $TIME_START, $TIME_STOP );

my $template = $self->load_tmpl( 'show_details.tmpl' );

$template -> param( 
    TABLE_HEADERS => [ map {{ VALUE => uc($_) }} @{$sth->{'NAME'}} ],
    RAW_DATA      => $sth,
);  

return $template -> output();
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)