I would suggest pulling the data from the DB as a hashref. There is a good HTML::Template and DBI example doing just this at HTML::Template Tutorial. To modify the data you would just loop through it:

my $ref = $sth->fetchall_hashref('key_field');
for my $key( %$ref ) {
    $ref->{$key}->{some_field} = modify( $ref->{$key}->{some_field} );
}

# pass modified hash to HTML::Template

sub modify {
   my $val = shift;
   # modify val
  return $val;
}
[download]