%hash = (
   key1 => 'bla',
   key2 => 0,
   key3 => undef,
);

print(  exist($hash{'key0'})?1:0, "\n");  # 0
print(defined($hash{'key0'})?1:0, "\n");  # 0
print(        $hash{'key0'} ?1:0, "\n");  # 0

print(  exist($hash{'key1'})?1:0, "\n");  # 1
print(defined($hash{'key1'})?1:0, "\n");  # 1
print(        $hash{'key1'} ?1:0, "\n");  # 1

# catch:
print(  exist($hash{'key2'})?1:0, "\n");  # 1
print(defined($hash{'key2'})?1:0, "\n");  # 1
print(        $hash{'key2'} ?1:0, "\n");  # 0

# catch:
print(  exist($hash{'key3'})?1:0, "\n");  # 1
print(defined($hash{'key3'})?1:0, "\n");  # 0
print(        $hash{'key3'} ?1:0, "\n");  # 0