use Time::Local;
my $file = "foo";

my($atime, $mtime) = (stat $file)[8, 9];
my($sec, $min, $hour, $mday, $mon, $year) =
    (localtime $mtime)[0..5];
my $on_hour = timelocal(0, 0, $hour, $mday, $mon, $year);

utime $atime, $on_hour, $file;

##</code><code>##

utime $on_hour, $on_hour, $file;