I would also suggest HTML::Template::Compiled, but your filter solution is pretty nice too. You can work around the ENV variable by building a filter based on the variable, using an embedded sub:

sub mk_include_filter {
    my $page = shift;
    return sub {
        my $text_ref = shift;
        $$text_ref =~ s/###/$page/g;
   };
}

my $template = HTML::Template->new(filename => 'layout.tmpl',
                                   filter => mk_include_filter('page.i
+nc') );
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