After following the constants through the program reduces to:

my $hideLetter;
my $restoreLetter;
{
  my $prevColor;
  $hideLetter = sub
    {
      $prevColor = ($_[0]->itemconfigure(qw(current -fill)))[4];
      $_[0]->itemconfigure(qw(current -fill black));
    };
  
  $restoreLetter = sub
    {
      $_[0]->itemconfigure(qw(current -fill), $prevColor);
    };
}


my $mainWindow = MainWindow->new( qw(-title jAPH) );
my $canvas = $mainWindow->Canvas(-width => '10c',
                 -height => '4.25c',
                 -relief => 'sunken',
                 -borderwidth => 2);
$canvas->createLine( [qw(2c    1c
                         3c    1c
                         2.5c  1c
                         2.5c  1c
                         2.5c  3c
                         2.25c 3.25c
                         2c    3c)], 
             -width => '.375c',
             -cap => 'round',
             -join => 'round',
             -tags => 'i',
             -fill => 'blue');
$canvas->createLine( [qw(3.5c  3.25c
                         4c    1c
                         4.5c  3.25c
                         4.25c 2.125c
                         3.75c 2.125c)],
             -width => '.375c',
             -cap => 'round',
             -join => 'round',
             -tags => 'i',
             -fill => 'red');
$canvas->createLine( [qw(5.25c 3.25c
                         5.25c 1c
                         5.75c 1c
                         6.25c 1.5c
                         5.75c 2c
                         5.5c  2.125c
                         5.25c 2.125c)],
             -width => '.375c',
             -cap => 'round',
             -join => 'round',
             -tags => 'i',
             -fill => 'green');
$canvas->createLine( [qw(7c 1c
                         7c 3.25c
                         7c 2.125c
                         7c 2.125c
                         8c 2.125c
                         8c 1c
                         8c 1c
                         8c 3.25c)],
             -width => '.375c',
             -cap => 'round',
             -join => 'round',
             -tags => 'i',
             -fill => 'yellow');

$canvas->bind( 'i', "<Any-Enter>",
           [ \&$hideLetter ] );
$canvas->bind( 'i', "<Any-Leave>",
           [ \&$restoreLetter ] );
$canvas->pack( -expand => 'yes',
           -fill => 'both' );
MainLoop();
[download]

__SIG__
printf "You are here %08x\n", unpack "L!", unpack "P4", pack
  "L!", B::svref_2object(sub{})->OUTSIDE;
[download]