Unfortunately, one has to be careful with DateTime::Duration objects' accessors: ->month only returns the "months" component of the difference. You usually want ->in_units instead, but note the limitations of conversion - for example, you can't convert a number of months to a number of days without knowing which months we're talking about (and that information is no longer available if we only have a duration), so you have to get both the months and days, and in_units will do the conversion from years to months for you.

use warnings;
use strict;
use DateTime;

my $dt1 = DateTime->new(year => 2013, month => 10, day =>  1);
my $dt2 = DateTime->new(year => 2018, month =>  3, day => 31);

my $dur1 = $dt2->subtract_datetime($dt1);
print $dt1->ymd, " to ", $dt2->ymd, ": ",
    $dur1->months, " months - oops!\n";

my ($days,$months) = $dur1->in_units('days','months');
print $dt1->ymd, " to ", $dt2->ymd, ": ",
    "$months months, $days days", "\n";

__END__

2013-10-01 to 2018-03-31: 5 months - oops!
2013-10-01 to 2018-03-31: 53 months, 30 days
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