#Start
use WWW::Mechanize::FireFox
#URL
$url = 'http://host/url/site';

#New instance
my $mech = WWW::Mechanize::FireFox->new();

#Exitkode
my $EXITCODE = 0;

#Get content of URL
$mech->get($url);
$str = 'javascript:InstallPrinter()';

eval_in_page_with_redefined_alert($mech,$str, sub { print "Caught alert(): @_" });
#$eval_in_page=>('javascript:InstallPrinter()');

#$mech->get('javascript:InstallPrinter()');

sub eval_in_page_with_redefined_alert {
    my ($self,$str,$redefined_alert) = @_;
    my $eval_in_sandbox = $self->repl->declare(<<'JS');
    function (w,str,myalert) {
        var unsafeWin = w.wrappedJSObject;
        var safeWin = XPCNativeWrapper(unsafeWin);
        var sandbox = Components.utils.Sandbox(safeWin);
        sandbox.window = safeWin;
        sandbox.window.alert = myalert;             // <--
        sandbox.alert = myalert;                    // <--
        sandbox.document = sandbox.window.document;
        sandbox.__proto__ = unsafeWin;
        var res = Components.utils.evalInSandbox(str, sandbox);
        return [res,typeof(res)];
    };
JS
    my $window = $self->tab->{linkedBrowser}->{contentWindow};
    my $uri = $self->uri;
    return @{ $eval_in_sandbox->("$uri",$window,$redefined_alert ) };
};