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 ) };
};
[download]

and call it as

eval_in_page_with_redefined_alert($mech,$str, sub { print "Caught aler
+t(): @_" });
[download]

If that doesn't work for you, something likely is wrong with my Javascript. You'll then have to look at the Mozilla FireFox Javascript documentation to figure out how things should be done instead.

MozRepl::RemoteObject: [Exception... "Illegal value"  nsresult: "0x800
+70057 (NS_ERROR
_ILLEGAL_VALUE)"  location: "JS frame :: chrome://mozrepl/content/repl
+.js -> file:///
C:/Documents%20and%20Settings/Administrator/Programdata/Mozilla/Firefo
+x/Profiles/kaq7
hscq.default/mozrepl.tmp.js :: anonymous :: line 5"  data: no] at C:\D
+ocuments and Se
ttings\Administrator\Skrivebord\PRINTLOOKER\firefox.pl line 40
[download]

I get this error when running this script:

#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 aler
+t(): @_" });
#$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 ) };
};
[download]

The following code works for me. Note that the interface is currently geared towards only capturing Javascript events (that is, the first passed argument) instead of properly capturing all arguments passed to the function. This will likely change in the next release of MozRepl::RemoteObject.

#!perl -w
use strict;
use Test::More;
use WWW::Mechanize::FireFox;

my $mech = eval { WWW::Mechanize::FireFox->new( 
    autodie => 0,
    #log => [qw[debug]]
)};

if (! $mech) {
    my $err = $@;
    plan skip_all => "Couldn't connect to MozRepl: $@";
    exit
} else {
    plan tests => 2;
};

isa_ok $mech, 'WWW::Mechanize::FireFox';

my $browser = $mech->tab->{linkedBrowser};
$mech->get_local('49-mech-get-file.html');

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 @{ $eval_in_sandbox->($window, $str, $redefined_alert ) };
};

my @alerts;
eval_in_page_with_redefined_alert($mech,"window.alert('Hello')", sub {
+ my @params = @_; push @alerts, \@params });
$mech->repl->poll;
is_deeply \@alerts, [['Hello']], "We can capture alert() boxes if we w
+ant to";
[download]

Update: With MozRepl::RemoteObject v0.08 and WWW::Mechanize::FireFox v0.09, you can now specify additional variables for the environment in which things get evaluated.