package Stopwatch;
use strict;
use warnings;
use Moose;

use DateTime;

has 'timer' => (
    is => 'rw', 
    isa => 'DateTime', 
    default => sub { DateTime->now }
); 

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

# ...

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