I feel you are mixing up arrays, slices and scalars a bit.
This is a slice and not an array element: @buscheckedi
However:

foreach $bus (reverse sort @busdir)
{

    $splbus = split (/_/, $bus, 1);
    
    if ($today >= $splbus)
    {
        $buschecked++;
    } 
}
[download]

assuming I got what you wanted to do. But as it looks now it might then be easier to use the Schwartzian Transform.

Have a nice day
All decision is left to your taste