Of course, the problem may be in the bitmap or icon files I'm using, so they're available at the following URLs:
FolderOpen.bmp
FolderOpenMask.bmp
FolderOpen.ico

#!perl/bin/perl
use warnings;
use strict;

use Win32::GUI;

my $main = Win32::GUI::Window->new(
  -name => 'Main',
  -width => 300,
  -height => 250,
);

my $il = Win32::GUI::ImageList->new(16, 16, 32, 1, 5);

# This shows the image, but it's not masked
$il->Add('FolderOpen.bmp', 'FolderOpenMask.bmp');

# As does this
#$il->AddBitmap(
#  Win32::GUI::Bitmap->new('FolderOpen.bmp', 0),
#  Win32::GUI::Bitmap->new('FolderOpenMask.bmp', 0),
#);

# And icon files don't work at all
#$il->AddBitmap(Win32::GUI::Bitmap->new('FolderOpen.ico', 1));

my $tv = $main->AddTreeView(
  -name => 'TreeView',
  -width => 280,
  -height => 200,
  -top => 10, -left => 5,
  -buttons => 1,
  -rootlines => 1,
  -imagelist => $il,
);

# Also, this doesn't seem to work, for some reason.
# The ImageList has to be passed when constructing the TreeView.
# Why?
#$tv->SetImageList($il, 24);

my $root = $tv->InsertItem(
  -text => 'Foo',
  -indent => 1,
  -image => 0,
);
$tv->InsertItem(
  -text => 'Bar',
  -indent => 1,
  -image => 0,
);
$tv->InsertItem(
  -text => 'Baz',
  -indent => 1,
  -image => 0,
  -parent => $root,
);

$main->Show(1);
Win32::GUI::Dialog();

sub Main_Terminate {
  return -1;
}
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