#!/usr/bin/perl -w
use strict;

use Tk;
use Tk::Compound;

  my $parent = MainWindow->new();
  my $b = $parent->Button;
  my $c = $b->Compound;
  $b->configure(-image => $c);
  $c->Line;
  $c->Bitmap(-bitmap => 'warning');
  $c->Space(-width => 8);
  $c->Text(-text => "Warning", -underline => 0);
  $b->pack(-side => 'left');
  
MainLoop();
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jdtoronto

Menubar doesn't need packing (did you even run my example code?)
Maybe you didn't have Tk:ToolBar installed - but check updated example code above which uses a builtin bitmap instead.

You use Tk::Compound widget on a Button widget but I'm using it on a command menuitem on a menubar.

Yes, but after I had responded. I see your issue, looking carefully at my menues I note the constant space to the left of the option text, or in your case the graphic. This space is used for the check mark when you use a checkbutton in the menu. There is also some small space to the right which does not seem controllable either. I did somemore experimenting with some of my own code, but in the end I did not find a way to do what you need. Here is your code but with a Checkbutton added so when you check it you can see some justification for the space.

#!/usr/bin/perl

use Tk;
use Tk::ToolBar;
use Tk::Compound;

#my $mw = tkinit;
my $mw = MainWindow->new();

#Load images from Tk::ToolBar.
my $tb = $mw->ToolBar();
$tb->destroy();
my $compound = $mw->Compound;
#How do we place this image to far left in menu?
$compound->Image( -image => 'acthelp16', -anchor => 'w' );
$compound->Space( -width => 8 );
$compound->Text( -text => "Open", -underline => 0 );

my $menubar = $mw->Menu( -type => 'menubar', );

#my $menuitem = [ [ command => Open, -image => $compound ] ];
$menubar->cascade(
  -label     => 'File',
  -tearoff   => 0,
  -menuitems => [ [ Button => 'Open', -image => $compound ],
                  [ Button => '~Quit', -command => [ destroy => $mw ] 
+],
                  [ Checkbutton => 'Checkthing' ],
                ]
);

$mw->configure( -menu => $menubar );

MainLoop;
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jdtoronto