Use fetchall_arrayref (instead of fetchall_hashref) to get an array of hashes right from the start: (untested)

my $rows = $sth->fetchall_arrayref({});

$_->{nw} = addComma( $_->{nw} )
    for @$rows;

$tmpl->param( items => $rows );

## Suitable for:
##
##     <tmpl_loop items>
##         <tmpl_var name>
##         <tmpl_var nw>
##         <tmpl_var number>
##     </tmpl_loop items>
[download]

You can also combine the prepare/execute/fetchall_arrayref into a single call to selectall_arrayref, if you use the Slice parameter. This is the simplest way to send query results directly into HTML::Template. Take a look at the excellent DBI recipes for more info.