use DBI;
use HTML::Template;

my $dbh = DBI->connent(
   qw(dbi:mysql:ndb user pass),
   {RaiseError => 1},
);

my $sites = $dbh->selectall_arrayref('
   select distinct site_name as name
   from sites 
   order by site_name
',{Slice=>{}});

for my $site (@$sites) {
   $site->{routers} = $dbh->selectall_arrayref('
      select distinct router as name
      from sites 
      where site_name = ?
   ',{Slice => 1}, $site->{name});

   #i removed the router = ? part from here
   #how do you expect to query a single router if there are multiples?
   $site->{interfaces} = $dbh->selectall_arrayref('
      select interface as name
      from sites 
      where site_name = ?
   ',{Slice => 1}, $site->{name});

}

my $tmpl = HTML::Template->new(filehandle => \*DATA);
$tmpl->param(sites => $sites);

print $tmpl->output;

__DATA__
<tmpl_loop sites>
   Name:    <tmpl_var name>
   Routers:
   <tmpl_loop routers>
      <tmpl_var name>
   </tmpl_loop>
   Interfaces:
   <tmpl_loop interfaces>
      <tmpl_var name>
   </tmpl_loop>
</tmpl_loop>