This sounds homework-y, so I'll get you started. Your OP is not so detailed so I'm grasping at straws for some of this. ;)
Step 1: Make a hash like this:

while(<>) {
    ($key, $value) = split(/==/);
    $hash{$key} = $value;
}
[download]

Step 2: lookup the string you want with your hash by using it as a key.
Step 3: print what the hash spits out as its value.