rules => [
  FOO => sub{print "here!"} # make this rule dependant on whether FOO has a child-tag called BAR?
]

##</code><code>##

rules => [
  FOO => sub{
    if(exists $_[1]->{'BAR'}){
      print "here!";
    }
  }
]

##</code><code>##

map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2
-$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20