You should pass the bg option when you instantiate the tree, not when you add it to the main window.

use warnings;
use strict;
use Tk;
use Tk::Tree;

my $main = MainWindow->new (-title => "Test tree");
my $tree = $main->ScrlTree
  (
  -itemtype => 'text', -separator  => '/',
  -scrollbars => "osoe",-bg => 'red'
  );

$tree->add
  (
  1,
  -text => 'Some sample text', -itemtype => 'text',
  );

$tree->autosetmode;
$tree->close (1);
$tree->pack(-fill=>'both',-expand => 1);

MainLoop;
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