my $TMPL = Template->new( {
    DELIMITER    => ';',
    INCLUDE_PATH => 'C:/Apache2/sites/foobar/template',

    INTERPOLATE  => 1
} );

$TMPL->process( \*DATA, {
    screen => qw(screen1 screen2)[rand 2]
} );

__END__
[% INCLUDE primary/html_head.tmpl %]
[% INCLUDE "$screen" %]
[% INCLUDE primary/html_foot.tmpl %]

[% BLOCK screen1 %]
This is screen #1.
[% END %]

[% BLOCK screen2 %]
This is screen #2.
[% END %]