Ick, duplicated code. :) You can use a hash, but why not an array?

my @pages = (
    'frontpage.html',
    'aboutus.html',
    'login.html',
);    # extra comma is there so adding new pages is easier

my $num = CGI::param('Num') || 1;
page($num - 1);        # or put a dummy at the start of @pages

sub page {
    $| = 1;
    my $num = shift;
    if (open(FILE, $pages[$num])) {
        print while <FILE>;
    }
}
[download]

A hash would work too, but if you're using numeric offsets already...

You could even make page 0 show stats.