One example of what anonymonk is getting at here (and yes, I've done this for legitimate purposes a few times that I can recall):

use warnings;
use strict;

package Fraud; {
    sub new {
        return bless {}, shift;
    }
    sub login {
        print "HA-HA, I'm not who you think I am!\n";
    }
}

package Trust; {
    sub new {
        return Fraud->new;
    }
    sub login {
        # recently was watching "That 70's Show"
        print "no login for you, dumbass\n";
    }
}

package main; {
    my $obj = Trust->new;
    $obj->login;
}
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