先月の1日〜末日までの日付を得る。
use Date::Calc qw(Add_Delta_YMD);
my ($sec, $min, $hour, $mday, $mon, $year, $wday, $yday, $isdst) = localtime(time());
$mon = $mon + 1;
$year = $year + 1900;
($sy,$sm,$sd) = Add_Delta_YMD($year,$mon,1,0,-1,0);
($ey,$em,$ed) = Add_Delta_YMD($year,$mon,1,0,0,-1);
$start_day = "$sm-$sd-$sy";
$end_day = "$em-$ed-$ey";