Finding the documentation is a bit tricky. You need to read the following paragraph in Tk::Tree:

The Tree class is derived from the HList class and inherits all the methods, options and subwidgets of its super-class.

And Tk::HList shows what we need to do:

Each list entry in an HList widget is associated with a display item. The display item determines what visual information should be displayed for this list entry. Please see Tk::DItem for a list of all display items.

Having read Tk::DItem, we can now solve the problem:

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

use Tk qw{ MainLoop };
use Tk::Tree;
use Tk::ItemStyle;

my $mw   = 'MainWindow'->new(-title => 'Tree');
my $tree = $mw->Tree->pack(-fill => 'both', -expand => 1);

$tree->add($_, -text => $_) for qw( orange orange.red orange.yellow );

my $blue = $tree->ItemStyle('imagetext', -foreground => 'blue');
$tree->itemConfigure('orange.red', 0, -style => $blue);
MainLoop();
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