I am not going to get into any specific but if you want to make a module out of it here is one way of doing it:

create a file with the .pm extenstion for example Error.pm
This goes inside:

package Error;

use Exporter;
@ISA = ('Exporter');
@EXPORT = ('handle_error');

sub handle_error{
    my $msg = shift ||' ';
    my $dbh = shift ||' ';
    my $sth = shift ||' ';

    #html::template - handle_error.tmpl
    my $tem = HTML::Template->new(filename => "handle_error.tmpl");
    $tem -> param(message => $msg);

    print $tem->output;

    #finish the state handle and disconnect, if necessary
    $sth->finish()     if $sth;
    $dbh->disconnect() if $dbh;
    exit 0;
}

1;
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in your calling script give the path to your newly created module Error.pm

use lib ("/home/z/xxx/www/modules/");
use Error;
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Now you can use your subroutine. Your subroutine probably won't work now, but hopefully you get the picture.