How would I go about and do that?

I've tried putting your code into my script and making the eval do the javascript:Install() part of it, without success.

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
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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]