my $style = $tree->ItemStyle(
    'imagetext',
    -foreground => 'black',
    -background => 'white',
    -selectforeground =>'white',
);

for my $top ( 1 .. 3) {
    $tree->add( $top, -text => $top );
    for ( 'A' .. 'C') {
        my $q = $tree->add( "$top.$_", -text => "$_" );
        $tree->entryconfigure("$top.$_", -style => $style);
    }
}