my $bit_10_9; #yes, this is a horrible abuse of globals
setbits( 0,0 );

my $bit_10_9_mb = $r2Frame->Menubutton(
    -textvariable => \$bit_10_9,
    -menuitems => [['command' => '01',
                    -command => sub{ setbits(0,1); }] ....

then

sub setbits
{
    $bit[10] = shift || 0;
    $bit[9] = shift || 0;
    $bit_10_9 = $bit_10 . $bit_9;
}