If he wants the date where his computer is located (as opposed to the date somewhere in Europe),

my $thirty_days_ago = DateTime->today(time_zone => 'local')->subtract(
+days => 30);
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Note that the above solution will fail two days a year in time zones with no midnight on a DST change. (There is at least one such time zone.) Workaround:

my $thirty_days_ago = DateTime->now(time_zone => 'local')->set_hour(12
+)->subtract(days => 30);
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