No, there's no way to do it using multiple keypresses (that I'm aware of).  And even if you could, there would be a problem when you held the 2 keys down, and relied on key repeats; whichever key you pressed "last" will dictate the direction that the object moves in.

You can try it yourself by holding down the left and up arrow, for example; the initial movement might appear to be a "wobbly" northwest movement, but after the repeat kicks in, the square will only move left or up.

My recommendation would be to assign a completely different set of keys.  For example, you could use the 8 keys centered around the "k":

    my $pbindings = {
        'Key-j'  => sub { $c->move($rect, -5,  0) },
        'Key-l'  => sub { $c->move($rect,  5,  0) },
        'Key-i'  => sub { $c->move($rect,  0, -5) },
        'comma'  => sub { $c->move($rect,  0,  5) },
        'Key-u'  => sub { $c->move($rect, -5, -5) },
        'Key-o'  => sub { $c->move($rect,  5, -5) },
        'Key-m'  => sub { $c->move($rect, -5,  5) },
        'period' => sub { $c->move($rect,  5,  5) },
    };
                                                                      
+          
    map { $mw->bind("<$_>" => $pbindings->{$_}) } (keys %$pbindings);
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