First, check for the existence of the hash key, then print the value of the hash key. Your "if" statement was performing an assignment and was probably always returning "true". Try this:
%hash = (one => 'uno', two => 'dos', three => 'tres');
$input = <STDIN>;
chomp $input;
if (exists $hash{$input}) {
print $hash{$input}, "\n";
}