c:\@Work\Perl\monks>perl -wMstrict -le
"my %h = qw(a 1  b 2  c 3  d 1);
 ;;
 my %r = reverse %h;
 keys %r == keys %h or die qq{hash @{[ %h ]} not unique};
"
hash c 3 a 1 b 2 d 1 not unique at -e line 1.