The problem has nothing in particular to do with Date::Manip. It's caused by something (from within ParseDate()) trying to modify the global $_, which is aliased to your tied log (something like $_ = "foo"; instead of the my $xxx = ParseDate($1); would have the same effect).   Rewrite your for-loop as

for my $t (@log)
{
    # ...
}
[download]

and everything should be fine.