You're already using Date::Calc, so I'd go with Add_Delta_Days:

#!/usr/bin/perl -w

use strict;
use warnings;
use Date::Calc qw{ :all };

my ($year, $mon, $day) = Today();
show_date("Today's date", $year, $mon, $day);

($year, $mon, $day) = Add_Delta_Days($year, $mon, $day, -30);
show_date("30 days ago", $year, $mon, $day);


sub show_date {
    my ($label, $year, $mon, $day) = @_;

    my $date = sprintf "%04d-%02d-%02d", $year, $mon, $day;
    print "$label:  $date\n";
}


__END__

[Output]
Today's date:  2013-06-07
30 days ago:  2013-05-08
[download]

Update:    Oh, now I see why you're getting different answers. It's because in:

$date[3] -= 4 * 7;
...
my $month_ago_3  = strftime('%Y-%m-%d', localtime(time - 4 * 7 * 86400
+));
[download]

You're subtracting 4 * 7 days (ie. 4 weeks), rather than 30 days.

Does that help?