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.