#
#  Replace ### with environmental variable variable 'page'
#
sub filter {
    my ($text_ref ) = shift;
    my $val = $ENV{'page'};
    $$text_ref =~ s/###/$val/g;
};


#
#  Load 'layout.tmpl' - and have that include 'page.inc'.
#
#  THis will expand the following in *both* files!
#
#  <!-- tmpl_var name='title' --> 
#
#
$ENV{'page'} = 'page.inc';
my $template = HTML::Template->new(filename => 'layout.tmpl',
                                   filter => \&filter );
$template->param( title => "Something here" );

print $template->output();