As far as I could get it from the docs (Tk::Canvas), and as I tried, the following should work: Substitute the canvas->createArc(...); lines by

$canvas->createArc($x1,$y1,$x2,$y2, -width=> 5, -fill=> 'purple',-outl
+ine=> 'purple',-style=> 'arc', -extent=> 59.59, -start => 300);
$canvas->createArc($x1,$y1,$x2,$y2, -width=> 5, -fill=> 'blue',  -outl
+ine=> 'blue',  -style=> 'arc', -extent=> 30,    -start => 270);
$canvas->createArc($x1,$y1,$x2,$y2, -width=> 5, -fill=> 'red',   -outl
+ine=> 'red',   -style=> 'arc', -extent=> 150,   -start => 120);
$canvas->createArc($x1,$y1,$x2,$y2, -width=> 5, -fill=> 'green', -outl
+ine=> 'green', -style=> 'arc', -extent=> 120,   -start => 0);
$canvas->createText($x_center+75, $y_center+75, -fill => 'black', -tex
+t => 'Foo Genome');
$canvas->createText($x_center+75, $y_center+90, -fill => 'black', -tex
+t => '25,632 bp');
[download]

Note the additional -start items.
To your second question: Have a look at the example at the end of The Perl/Tk User's Guide.
Hope this helped.