if (-d $pathname) {
  # $pathname exists and is a directory
  # ...
}
elsif (-e _) {
  # $pathname exists, but is NOT a directory
  # ...
}
else {
  # $pathname doesn't even exist
  # ...
}

##</code><code>##

if (-d $d) {
  open FILE, "$d/$f" or die "Open failed";
}
else {
  die "No directory $d";
}

##</code><code>##

open FILE, "$d/$f" or die "Open failed";