If by width of the screen you mean just the window itself, that's easy:

$c->pack(-expand => 1, -fill => 'both');
[download]

But if you mean, as I think you do, the width of the monitor screen, try using the methods vrootheight and vrootwidth as well:

#!/usr/bin/perl -w

use Tk;
use strict;    

my $mw = MainWindow->new;
my $vh = $mw->vrootheight;
my $vw = $mw->vrootwidth;

# Note that the 'virtual window' height and width are $vh and $vw
# respectively, so we use those dimensions for our Canvas height 
# and width, and let the Canvas expand and fill in both x and y
# directions.
#
my $c = $mw->Canvas(
-width => $vw, 
-height => $vh, 
-background =>'blue', 
-scrollregion =>[0, 0, 100, 100]); 
$c->pack(-expand => 1, -fill => 'both');

$c->createRectangle(100, 100, 150, 150, -fill => 'yellow'); 

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

my $main = MainWindow->new(-title => "Resize Test");
$main->geometry("600x400+100+100");

$main->Button(
   -text => "Exit Program",
   -command => sub { exit }
  )->pack();

my $button = $main->Button(
         -text => "Go To Fullscreen",
         -command => \&resizer,
  )->pack();

MainLoop; 

sub resizer{ 

  if ( $button->cget(-text) =~ /Fullscreen/ ){
  
     $main->geometry($main->screenwidth . 'x' .
             $main->screenheight . '+0+0');
     $button->configure(-text=>"Go to Window");
    }else{
      $main->geometry("600x400+100+100");  
      $button->configure(-text=>"Go To Fullscreen");

    }

}
[download]

Ability to do this aside, keep in mind that grabbing the entire screen could be considered anti-social behavior by some. If I want your window to take up the entire available real estate I'll tell my window manager to make it so.

(Of course there's exceptions, things like VNC clients that are representing the entire remote display in a special full screen mode; but those are few and far between.)