There's always the Orcish Manouvre. Simply keep around a hash of template names and lazily evaluate them if necessary:

my %templates;

# some code

sub include {
    my $tpl = shift;
    $templates{$tpl} ||= fetch($tpl);
    return $tpl;
}

sub fetch {
    my $tpl = shift;
    local (*TPL, $/);
    open(TPL, "template_folder/$tpl") or return;
    return <TPL>;
}
[download]

It may look very different, and you can get away with tieing the hash to do lazy lookups. The important point is that, if you have a persistent process that may need to read several files, open them only when necessary and store them all other times.

If your process isn't persistent, then there's not a lot you can do. Open them only when you need to, and pray to the virtual memory subsystem that they stay in cache.