my %start= qw( [( )] [| |] [{ }] );
my %end= reverse %start;
my $expect= "";   # Closing delimiter we expect, if any.
my $code;
for my $chunk (  split m#( \[ [(|{] | [)|}] \] )#x, $template  ) {
    if(  "" eq $expect  ) {
        if(  $start{$chunk}  ) {
            $expect= $start{$chunk};
            $code= "";
        } else {
            warn "Unmatched $chunk\n"   if  $end{$chunk};
            print $chunk;
        }
    } else {
        if(  $chunk eq $expect  ) {
            print expand( $code );
            $expect= "";
        } else {
            $code .= $chunk;
            warn "Found $chunk inside $end{$expect} $expect block\n"
              if  $start{$chunk}  ||  $end{$chunk};
        }
    }
}