use POSIX 'ceil';

    my @things    = 1 .. 42;
    my $per_page  = 20;
    my $num_pages = ceil( @things / $per_page );
    my $page      = param('page') || 1;

    ## verify page num is within range
    $page = 1          if $page < 1;
    $page = $num_pages if $page > $num_pages;

    my $first = ($page-1) * $per_page;
    my $last  = $first + $per_page - 1;
    $last     = $#things if $last > $#things;

    print @things[ $first .. $last ];