fu_moose.pl

#! /usr/bin/perl

package WTF;
use Moose;

has a => (
  is      => 'ro' ,
  isa     => 'Str' ,
  default => sub { shift->b() . ' harder' },
);

has b => (
  is       => 'ro',
  isa      => 'Str' ,
  required => 1 ,
);

package main;
use feature 'say';

my $wtf = WTF->new( b => 'fuck me' );

say $wtf->a;

output

% for i in `seq 1 10` ; do ./fu_moose.pl ; done
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder
fuck me harder
fuck me harder
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder
fuck me harder
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder
fuck me harder
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder
Use of uninitialized value in concatenation (.) or string at ./fu_moose.pl line 9.
 harder