use CGI qw(param upload); # and whatever else you want to import
...
for my $key (param()) {
  next unless $key =~ m{^img\[(.*)\]$};
  my $image_key = $1;
  my $image_filename = param($key);
  my $image_handle = upload($key);
  # do what you want with $image_key and $image_filename and $image_handle
}