use Tk;
use Tk::Font;

my $mw = MainWindow->new;

my $label = $mw->Label(
   -text => "Hello",
   -font => "Times 12"
)->pack(-expand => 1, -fill => 'both');

$label->bind('<Configure>', \&ResizeText);

MainLoop;

sub ResizeText
{
   my ($widget) = @_;
   $widget->bind('<Configure>', "");

   ## Check to see if the font is "registered"
   ## so that we can use Tk::Font methods reliably

   my $knownFont = 0;
   my $font = $widget->cget('-font');
   foreach my $f ($widget->fontNames()) {
      if ($$f eq $$font) {
         $knownFont = 1;
         last;
      }
   }

   ## If we're dealing with an unregistered font we
   ## could run into problems, so it's safest to create
   ## a new font based off the unregistered fonts attrs.
   ## Otherwise, reuse the existing font rather than
   ## recreating a new one.
   
   unless($knownFont) {
      $font = $widget->Font($font->actual);
      $widget->configure(-font => $font);
   }

   my $widthF = $font->measure($widget->cget('-text'));
   my $heightF = $font->metrics('-linespace');

   my $xRatio = $widget->width / $widthF;
   my $yRatio = $widget->height / $heightF;
   my $minRatio = $xRatio < $yRatio ? $xRatio : $yRatio;

   my $fontSize = $font->actual('-size');
   $font->configure(-size => ($minRatio * $fontSize));
   
   $widget->update;
   $widget->bind('<Configure>', \&ResizeText);
}