From language/variables

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The constant prefix declares that the value it tags is not going to change during its lifetime.

constant $pi2 = pi * 2;
$pi2 = 6; # OUTPUT: «(exit code 1) Cannot assign to an immutable value␤ 

The value is assigned at compile time. Since Raku modules are precompiled automatically, constants defined in modules are not re-evaluated when the program is run. Please check the section on constants in the Terms page for additional information.