I just tried the following on ActivePerl on Win32.

use Tk;

my $mw = Tk::MainWindow->new;
my $txt = $mw->Label(-text=>"Value:");
$txt->pack;
$mw->Button(-text=>"Ok", -command=>sub {$txt->configure(-text=>"Value:
+ " . rand)})->pack;
srand (time | $$);
$mw->MainLoop;
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And I don't see any pattern. Perhaps a bit more detail would help?

Update: Note that in my example I only call srand once. I tried it again with the srand call inside the callback function and I got a similar result to yours.