my $page = url_param('page');
my $top = $page* 20);
my $bottom = $top - 20;

if ($page ne "") {
for ( grep defined($_), ( reverse keys %upload )[ $bottom .. $top ] ) {
do this if url_param exists ...
}
else {
for ( grep defined($_), (reverse keys %upload )[ 0 .. 19] ) {
do this if on first page, without url_param ...
}