From type/Numeric

See Original text in context

multi sub    log2(Numeric:D)
multi method log2(Numeric:D:)

Calculates the logarithm to base 2. Returns NaN for negative arguments and -Inf for 0.

From type/Cool

See Original text in context

multi method log2()
multi sub log2(Numeric $x)
multi sub log2(Cool    $x)

Coerces the invocant to Numeric, and returns its Logarithm in base 2, that is, a number that approximately (due to computer precision limitations) produces the original number when 2 is raised to its power. Returns NaN for negative arguments and -Inf for 0.

say log2(5);            # OUTPUT: «2.321928094887362␤» 
say "4".log2;           # OUTPUT: «2␤» 
say 4.log2;             # OUTPUT: «2␤»