Henco, nice work. I agree with Gummo and Sean that you should have used the sockmark command instead of the global var. There are a couple of overlooked considerations:

1) Make sure the $2 entered is a URL, not something else.

2) you should add a trigger protection to it to safeguard the client running the code to not get flooded.

Here is the improved version of your script:

on $*:text:/^!url\s((\w{3,8}\72\/\/)?(w{3}\56.+))/iS:#:{
  if !%tc { inc -u4 %tc | if $sock(lop) { sockclose lop }
  sockopen lop www.l0p.us 80 | sockmark lop $chan $regml(1) }
}
on *:sockopen:lop:{
  var %sockmark $gettok($sock(lop).mark,2,32)
  sockwrite -n lop GET $+(/api-make.php?alias=&url=,%sockmark) HTTP/1.1
  sockwrite -n lop Host: www.l0p.us $+ $str($crlf,2)
}
on *:sockread:lop:{
  var %read | sockread %read
  if http:// isin %read { .msg $gettok($sock(lop).mark,1,32) Shortened URL: $v2 }
}