How can I modify a Perl DateTime object?

In my script I need to make a cycle like this one:

use DateTime;
for $j(0..3){
    my ($date) = DateTi开发者_如何转开发me->now->ymd;
    my ($k) = 0;
    while($k <= $j){
        $date = ($date->subtract( days => 7));
        $k++;
    }
print "$date\n";
}

which should get the current date, then one week ago, etc. Sadly, after getting the correct current date, it doesn't work and I don't know what's wrong.

Error message is "Can't call method "subtract" without a package or object reference [...]", 

but I have no idea how to fix this.

If possible, I'd like to keep using DateTime only OR replacing it with another module (possibly no more than one).

Datetime->now->ymd is a scalar (string, it appears), not an object/reference. You can't call subtract on it because it doesn't exist. You'll probably just want to try omitting the ymd part when you assign to $date:

my ($date) = DateTime->now;
...

for(0..$j) {
    $date = ($date->subtract( days => 7));
}

...

If you want to access the ymd value, do it after you've created the object:

my ($date) = DateTime->now;
...
my ($ymd) = $date->ymd;

See the CPAN page for more.

If you tried using Data::Dumper on $date you would immediately see what the problem is: $date is a string, not an object. You should delay calling ymd() on it until you need to stringify it.

use strict;
use warnings;

use DateTime;

for my $j (0..3)
{
    my $date = DateTime->now;
    $date = $date->subtract(days => 7) for (0 .. $j);
    print $date->ymd, "\n";
}

PS. This code can be simplified even more by combining the loops:

my $date = DateTime->now;
for my $weeks (0..4)
{
    print $date->ymd, "\n";
    $date = $date->subtract(days => 7);    
}

It's better to do the ymd when printing, after the calculations and working on the Date object.

See the difference when switching when ymd and subtract are called:

#!/usr/bin/perl -w    

use strict;    
use Data::Dumper;
use DateTime;

   sub main{   
      my $date = DateTime->now->subtract(days=>7)->ymd;
      print "$date\n\n";
   }    

   main();