$ cat t
my %h = (a=>1, b=>2, c=>3);

print "a ", exists($h{a}) ? "exists" : "does not exist", "\n";
print "c ", exists($h{c}) ? "exists" : "does not exist", "\n";
print "f ", exists($h{f}) ? "exists" : "does not exist", "\n";

$ perl t
a exists
c exists
f does not exist