Hello,
if two values in hash are undef, it is recognized as a difference.
Example:
use Hash::Diff qw(diff);
my %a = ('bar' => undef);
my %b = ('bar' => undef);
my %c = %{ diff(\%a, \%b) };
# %c = %{ bar => undef }
Tue Aug 25 00:28:50 2015bo [...] startsiden.no - Taken
Thu Aug 27 04:26:53 2015bo [...] startsiden.no - TimeWorked changed from (no value) to '10'
Thu Aug 27 04:26:54 2015bo [...] startsiden.no - Status changed from 'new' to 'resolved'
Thu Aug 27 04:26:55 2015bo [...] startsiden.no - Fixed in 0.007 added