This short programme demonstrates how to change font size. I hope it can help you.
#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::Font;
sub change {
my ($size_ref, $change, $l) = @_;
$$size_ref += $change;
$l->configure(-font => $l->fontCreate(-size => $$size_ref));
}
my $mw = 'MainWindow'->new(-title => 'Scalable');
my $size = 12;
my $font = $mw->fontCreate(-size => $size);
my $l = $mw->Label(-text => 'Some text', -font => $font)->pack;
my $f = $mw->Frame->pack;
my $plus = $f->Button(-text => '+', -command => [\&change, \$size, 1
+, $l])->pack(-side => 'left');
my $minus = $f->Button(-text => '-', -command => [\&change, \$size, -1
+, $l])->pack(-side => 'right');
my $quit = $mw->Button(-text => 'Quit',
-command => sub { exit })->pack;
MainLoop();