my $mech = WWW::Mechanize->new();
$mech->agent_alias( 'Firefox' );
$userid  = "usr";
$password = "password";
$mech->get( "https://login.binck.nl/klanten/Login.aspx?ReturnUrl=%2fkl
+anten%2fdefault.aspx" );
$mech->submit_form(
    form_number => 2,
    fields => {
        userid => "$userid",
        password => "$password"
    },
    button => "btnSignon"
);
[download]

I put strict off and it gives no errors,

It is recommended to put it back on and declare variables properly.

then i ran it and it gave line 6 unknown agent 'firefox'

This is because there is no agent alias called "Firefox". Use known_agent_aliases() to get available names, for example:

$ perl -MWWW::Mechanize -E'say join ", ",WWW::Mechanize::known_agent_a
+liases'
Linux Konqueror, Linux Mozilla, Mac Mozilla, Mac Safari, Windows IE 6,
+ Windows Mozilla
[download]

and line 1900 of mechanize.pm says there is no form numbered 2

Try using Data::Dumper and forms() method to get the list of forms available for WWW::Mechanize.
Sorry if my advice was wrong.

Or just $mech->dump_forms;