For example:

use 5.010;
use strict;
use warnings;

BEGIN {
    # Allow false to be modified.
    &Internals::SvREADONLY(\!!0, 0);
    
    # Modify false to be equal to 1.
    ${\!!0} = 1;
    
    # Disallow false to be modified again.
    &Internals::SvREADONLY(\!!0, 1);
};

say "left is right!"   if "left" eq "right";
say "up is down!"      if "up" eq "down";
say "black is white!"  if "black" eq "white";
say "maths is flawed!" if 7 > 100;

[download]
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Perl allows you to change the definition of false to make it true. (And vice versa.) by tobyink

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