Something like (untested):

$UP=-1;
$DOWN=1;

sub move
{
my $lb=shift;
my $direction=shift;
my $cur=$lb->curselection();
$cur+=$direction;
my $size=$lb->size();
$cur=0 if $cur>=$size;
$cur=$size-1 if $cur<0;
$lb->see($cur);
}
[download]

usage: move($lb,$UP); move($lb,$DOWN); (only works if one and only one element is selected)