You are reading the file into @prep, but are not really searching @prep for the pattern.

$tag =~ m/<!--..? -->/;

This line searches for the pattern in the variable $tag, which may or may not have been defined (considering it is a global, not a lexically scoped, variable). Consider the following instead:

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

I have not attempted above to change your algorithm or data structure, although I would use a hash containing references to the subroutines, like:

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

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

Also, I would not read the whole file at once, but instead:

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
[download]

HTH,
/prakash

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