Yeah, that was just lazyness, sorry. You can substitute the $le by $_[0]->parent.

You can see what is inside $_[0] by checking @_:

#!/perl
use strict;
use warnings FATAL => qw(all);

use Tk;
use Tk::LabEntry;
use Data::Dumper qw/Dumper/;

use List::Util qw(first); 
use Scalar::Util qw(looks_like_number);

my $mw = MainWindow->new();
$mw->title("Test");
my $test = 8;
my $width = 250; my $length = 125;
$mw->minsize($width, $length);

my $FONT = $mw->fontCreate(-family => 'verdana', 
     -size => 14, -weight => 'normal');

my $le = $mw->LabEntry(-label => 'Value',
           -labelPack => [qw/-side left -anchor w/],
       -labelFont => '9x15bold', 
           -font => $FONT,
       -relief => 'ridge',
           -textvariable => \$test,
       -width => 2,
 )->pack();

$le->bind('<Key>' => sub { labelCheck($_[0]);});

MainLoop;

sub labelCheck
{
  print Dumper \@_;
  my $x = $_[0]->get(); 
  if ( !(looks_like_number($x)) or ($x < 0) )
  {
    $_[0]->delete(0, 'end');
    my $label = $_[0]->parent->Subwidget('label');
    $label->configure( -background => 'red');
    # $_[0]->configure( -labelBackground => 'red');
  }
  else
  {
    $_[0]->configure( -background => '#f0f0f0',);
    # $_[0]->configure( -labelBackground => '#f0f0f0');
    do_something();
  }
  return 1; 
}

sub do_something { print $test, $/; }
[download]

For me, this returned:

$VAR1 = [
          bless( {
                   '_TkValue_' => '.labentry.entry',
                   '_XEvent_' => bless( do{\(my $o = '   $   ÿÿÿÿü˜ï
+¬
œ¿ê    Tç®       t
  û      A   
   a   
               A   (ø \
+'ààï ù a   ¼èˆ ä"¶ÔŒ')}, 'XEvent' )
                 }, 'Tk::Entry' )
        ];
[download]

It's a Tk::Entry inside a LabEntry (cf. .labentry.entry).