You typically only need to put the tag in the binding if it refers to a window. Basically, just removing the $lstbox tag from the bind to <Delete> command makes it work.

BTW, it is usually better if you can provide a complete runable script when you ask a question. A lot of times, if there isn't a framework to run your code, your question will get fewer responses.

I built a small demo script using your posted code. Try this.

Update: Whoops, didn't notice extended select mode. Changed Deltxt sub appropriately.

use strict;
use warnings;

use Tk;

my $testlbl = 15;
my $font    = '{New Times Roman} 12';

my $p2 = MainWindow->new;

$p2->Label(
    -width      => $testlbl,
    -text       => "Parameter List",
    -foreground => '#FFFFFF',
    -background => '#666666',
    -font       => $font,
    -anchor     => 'center'
  )->grid(
    -row        => 6,
    -column     => 0,
    -columnspan => 5,
    -padx       => 5,
    -pady       => 0,
    -sticky     => 'ew'
  );

my $lstbox = $p2->Scrolled(
    "Listbox",
    -scrollbars => 'osoe',
    -selectmode => 'extended',
    -foreground => '#FFFFFF',
    -background => '#7E7E68',
    -font       => $font,
    -width      => 40
  )->grid(
    -row        => 7,
    -column     => 0,
    -columnspan => 5,
    -padx       => 5,
    -pady       => 0,
    -sticky     => 'nsew'
  );

$lstbox->Subwidget("corner")->configure( -background     => '#666666' 
+);
$lstbox->Subwidget("xscrollbar")->configure( -background => '#666666' 
+);
$lstbox->Subwidget("yscrollbar")->configure( -background => '#666666' 
+);
$lstbox->bind( '<ButtonRelease-1>' => sub { $lstbox->focus } );
$lstbox->bind( '<Delete>' => \&Deltxt );

$lstbox->insert( 'end', $_ ) for ( 1 .. 20 );

MainLoop;

sub Deltxt {
    return unless my @indicies = $lstbox->curselection;
    $lstbox->delete( $_ ) for @indicies;
}
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