package Stopwatch;
use strict;
use warnings;
use DateTime;

use Class::MethodMaker [ 
  scalar => [ { -type => 'DateTime', -default_ctor => sub { DateTime->now },             
              'timer' ],
  new    => 'new',
];

sub diff {
    my $self = shift;
    $self->timer - DateTime->now;
}

# ...

my $sw = Stopwatch->new;
# ... wait a moment
print $sw->diff;