$template =~ s[
        \[% \s+ IF \s+ (\w+) \s+ %\] # $1 var
        (.*?)                        # $2 then
        \[% \s+ END \s+ %\]
    ][do {
    my($a, $b, $c) = ($1, $2, '');
    if ($b =~ /[% \s+ ELSE \s+ %]/gsx) {
#==============^                ^      backslashes removed!
        my(@c) = split(/[%\s+ELSE\s+%]/s, $b, 2);
#=======================^            ^ backslashes removed!
        warn 'c(', join('|', @c), ')' if DEBUG;
        $b = $c[0];
        $c = $c[1];
    }
    warn "a($a) b($b) c($c)\n----\n" if DEBUG;
    defined $$v{$a} && $$v{$a} ? $b : $c;
};]egsx;