Bad naming practices, illustrated

Foo-on-all-this

#!/usr/bin/env perl
## Some programs exist merely to serve as an object lesson to others...

package Foo {
our $F = qq{"Life, the Universe, and Everything" by DA};
our $O = 'Off by one error';
}

$f = ' is';
$o = ' the';
$o_= ' answer';

$foo = &foo($Foo::F);
push @foo, --$foo; $foo++;
push @foo, ++$foo; $foo--;
$foo{$foo}               = $Foo::F;
$foo{$foo[$#foo]}        = $Foo::O;
$foo{$foo[$foo ne $foo]} = $Foo::O;
sub foo { split '', shift }

$foo_ .= "$foo$f$o$o_\n";
$foo_ .=  $foo{$foo[$foo/$foo]} . "\n";
$foo_ .=  $foo{$foo[$foo-$foo]} . "\n";
$foo_ .=  $foo{$foo} . "\n";

print $foo_;

my $FOO = <<'FOO';
42 is the answer
Off by one error
Off by one error
"Life, the Universe, and Everything" by DA
FOO

use Test::More;
is ($foo_, $FOO);
done_testing();

Naming_conventions

Verbiage from Rosetta-Code 'Naming_conventions' task:

This being Perl, you are of course given wide lattitude in the names you can use for your program elements.
But it can be helpful to other programmers, and possibly your future self, to use letter case and underscores 
to indicate the scope or nature of a variable, so their roles can be understood at a glance.
Some generally accepted conventions:

    $ALL_CAPS_HERE   constants 
    $Some_Caps_Here  package-wide global/static
    $no_caps_here    function scope my/our/local variables

Note the use of underscores for readability.
Subroutines and variables meant to be treated as private are prefixed with an underscore. 
With all-caps constants, be careful for conflicts with Perl special variables.
Function and method names should be all lowercase.

Any reasonably modern version of Perl can use Unicode characters in names, so employ them where it makes sense. It 
may be necessary to invoke the <code>use utf8</code> pragma such cases.

<b>A caution about variables names and sigils</b>:
Perl won't stop you from giving three variables in the same scope the names
<code>%foo</code>, <code>@foo</code> and <code>$foo</code>, but that doesn't 
mean it's a good idea.