This is not technically constant, but is the most common way of doing it. The third-party library Alexandria provides define-constant that could define a constant and one could also use define-symbol-macro from the standard library
PLANET = "Earth"
Not a constant but closest implementation
NSString * const Planet=@"Earth";
There's a (weak) convention to keep constants capitalized
Module Const::Fast is preferred over Readonly, which is greatly preferred over 'use constant'. Google "Perl Critic Policy Prohibit Constant Pragma" and read the Const::Fast documentation for more details.
static final planet = "Earth"
static final planet = 'Earth'
static final planet = /Earth/
static final planet = '''Earth'''
static final planet = """Earth"""