Your problem may be solved if you don't try to match the entire URL, but only the part you're interested in, before any colons or slashes:

sub getDomain {
  my ($url) = @_;
  $url =~ m#(http://[^/:]+)#i;
  return $1;
}
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This doesn't have exactly the same functionality as your function (specifically, it chops the trailing slash), but that's easy to modify if you want. You may also want to look into the Perl module called "URI".

Update: changed return $url; to return $1;.