use warnings;
use strict;

package Device; {
    sub new {
        my $self = bless {}, shift;

        $self->{s1} = Sensor->new;
        $self->{s2} = Sensor->new;
        $self->{s3} = "string";

        return $self;
    }
    sub event {
        my ($self) = @_;

        for (keys %{ $self }){
            if (UNIVERSAL::can($self->{$_}, 'motion')){
                print "yep, '$_' is a sensor object\n";
            }
            else {
                print "'$_' isn't a damned sensor!\n";
            }
        }
    }
}

package Sensor; {
    sub new {
        return bless {}, shift;
    }
    sub motion {
        ...
    }
}

package main;

my $device = Device->new;

$device->event;

##</code><code>##

yep, 's1' is a sensor object
's3' isn't a damned sensor!
yep, 's2' is a sensor object