The following seems to work correctly:

sub check_card
{
    my $cardno = $card->get();

    if    (length $cardno == 0)
    {
        error_no_card();
    }
    elsif (length $cardno >  0)
    {
        $mw->Label
        (
            -text  => "\nChecking Account Details for '$cardno'.\n Ple
+ase Wait......\n",
            -width => 54,

        )->pack();

        $mw->update;

        my $exists = check_in_SQLStore($cardno);

        print "We got nothing\n" unless $exists;
    }
}
[download]

That is, call $mw->update; immediately after creating the label.

See, for example, the recent thread Perl/Tk window contents disappear when obscured then revealed.

Hope that helps,