3.2.34 exists

exists EXPR

This function returns true if the specified hash key exists in its hash, even if the corresponding value is undefined.

print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};

A hash element can only be true if it's defined, and can only be defined if it exists, but the reverse doesn't necessarily hold true in either case.

EXPR can be arbitrarily complicated as long as the final operation is a hash key lookup:

if (exists $ref->[$x][$y]{$key}) { ... }