use constant CONSTANT => 10;;

Internals::SvREADONLY( my $READONLY = 10, 1);;
[Type of arg 1 to Internals::SvREADONLY must be one of [$%@] (not scalar assignment) at (eval 11) line 1, near "1)"

$READONLY = 10; Internals::SvREADONLY( $READONLY, 1);;
++$READONLY;;
[Modification of a read-only value attempted at (eval 13) line 1, <STDIN> line 6.

cmpthese -1,{ 
    a=>q[ for(1..1000){ my $string = "the constant is " . CONSTANT . "\n" }  ],
    b=>q[ for(1..1000){ my $string = "the readonly is $READONLY\n"; }        ],
};;
    Rate    b    a
b 3893/s   -- -46%
a 7236/s  86%   --