From type/Blob

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multi method decode(Blob:D: $encoding = self.encoding // "utf-8")

multi method decode(Blob:D: $encoding, Str :$replacement!,
                    Bool:D :$strict = False)

multi method decode(Blob:D: $encoding, Bool:D :$strict = False)

Applies an encoding to turn the blob into a Str; the encoding will be UTF-8 by default.

my Blob $blob = "string".encode('utf-8');
say $blob.decode('utf-8'); # OUTPUT: «string␤»

On malformed utf-8 .decode will throw X::AdHoc. To handle sloppy utf-8 use utf8-c8.