Please 'use warnings;', that would have brought you the following warnings:

Name "main::d" used only once: possible typo at ./t7.pl line 16.
Can't locate object method "user" via package "strict" (perhaps you fo
+rgot to load "strict"?) at ./t7.pl line 3.
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And $d in the first warning is the problem, it is never set, instead you are setting $c twice

The other warning is because you wrote 'user' instead of 'use'

PS: Your sort line might be better written as my @ordered = sort compare  @dates;. As you have it, sort calls the "block-subroutine" which in turn calls the compare subroutine. It is possible that perl optimizes this away but I wouldn't bet on it.