use File::Find;

#...

sub dir_has
{
  my $base=shift;
  my $reg=shift;
  my $ok=0;
  find(sub{ $_=~$reg && $ok++ },$base)
  return $ok;
}

rmtree($server."\\".$folder) if (dir_has("$server\\$folder",$regex));

use File::Spec;
my $path=File::Spec->join($server,$folder);

rmtree($path) if (dir_has($path,$regex));