perl hash functions

Hello,

Does anyone know how to use the Perl hash function "exists"?
The book I'm using says there is a hash function as follows:

exists $hash{$key} which returns true if the key is found in the hash table.

I want to do something like:

if (exists $hash($key})
{do something ...}
else
{do something else...}

I keep getting errors. Does anyone know the proper syntax for this function? Thanks.

Bob