$_ = q{ ... wall text ...};
my $salt = 'rD';
# Replace every character in the wall with
# the third character from crypting that character.
s/(\S)/substr crypt( $1, $salt ), 2, 1/ge;

# Ignore the white space,
# and replace hex digit pairs with
# the ASCII characters they encode.
my $prog = '';
for ( split ) {
    s/(..)/chr hex $1/ge;
    $prog .= $_;
}

# Eval the resulting program.
eval $prog;