$page = join "", @prep; # since the lines are in an array
if (my ($tag) = $page =~ m/<!--(.+) -->/) {
    # call the appropriate sub based on the tag
    if ($tag eq 'a') {
        suba; exit;
    } elsif ($tag eq 'b') {
        subb; exit;
    } # ...etc...
}

##</code><code>##

my $subs = {
    a => \&suba,
    b => \&subb,
    # etc.
}

if ($subs->{$tag}) {
    $subs->{$tag}->();
} else {
    warn "Unknown tag <$tag> in file <$prepage>\n";
}

##</code><code>##

my $tag;
while (<PAGE>) {
    if (m/<!--(.+) -->/) {
        $tag = $1;
        last; # assuming there is only one tag in each file
    }
}
close <PAGE>;
# proceed to call the tag handler