Use the ref() funtion...

if (ref($array[$i]) eq "ARRAY") {
  print "It's an array!";
}
else {
  print "Not an array!";
}
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Update: Oops, you said in a hash, my bad... here's a better example:

if (ref $hash{$key} eq "ARRAY") {
  print "It's an array ref!";
}
else {
  print "Not an array ref!";
}
[download]

Alan "Hot Pastrami" Bellows
-Sitting calmly with scissors-