>perl -wMstrict -le
"my %hash = qw(one 1  uno 1  un 1  two 2  tres 3);
 my %hsah = reverse %hash;
 ;;
 for my $v (1, 2, 4) {
   print qq{value '$v' },
         exists $hsah{$v} ? qq{is $hsah{$v} } : q{does not exist },
         q{in original hash};
   }
"
value '1' is uno in original hash
value '2' is two in original hash
value '4' does not exist in original hash