# TOTALLY UNTESTED!!
no warning 'redefine';

my %fhs;

BEGIN {
        *CORE::GLOBAL::open = sub { 
          my ($fh,$mode,$path) = @_;
          
          # open the file
          
          $fhs{fileno($fh)} = $path.' '.${^LAST_FH};
      };
    }


END{
     print "Files already opened:\n", map {"fileno $_ $fhs{$_}\n"} sort keys %fhs;
}