$\=$,=$/;
while($n=2*<>){
$_=' 'x--$n;$_="/$_/$_ \\";@_=();
s! \\!\\!,@_=($_,@_,y!/\\!\\/!r),s!^! !while/ \\/;
s!^/\K *| *(?=\\/)!'_'x$n!e for@_;
print' 'x($n/2+1).'_'x++$n,@_}
Note that non-ascii characters in the above source code will be escaped (such as \x9f).