A few ideas:

• A few enums could make sense in this situation, depending on what syntax you are aiming for
• I'm not sure that you need to specify the associativity and precedence when they are the same as the default for those operations
• A postcircumfix operator would allow you to send things like 'kg' as a parameter
• I'm not sure you need to list the submethod BUILD since it behaves like the default
• You can avoid some redundancy in defining your operators if Mass and Length have a common ancestor

Here's another version that accomplishes something similar:

enum Prefixes (
    :pico(10.0 ** -12), :nano(10.0 ** -9), :micro(10.0 ** -6),
    :milli(10.0 ** -3),
    :centi(10.0 ** -2), :one(1.0), :hecto(10.0 ** 2),
    :kilo(10.0 ** 3),
    :mega(10.0 ** 9), :giga(10.0 ** 9), :tera(10.0 ** 12),
);

enum MassAbbrevs (
    :g(one),
    :kg(kilo)
);

enum LengthAbbrevs (
    :m(one),
    :km(kilo)
);

class SI { }

class Mass is SI {
    has $.v;
}

class Length is SI {
    has $.v;
}

multi postcircumfix:<[ ]>(Real $n, MassAbbrevs $abbrev ) {
    return Mass.new(v => $n * $abbrev);
}

multi postcircumfix:<[ ]>(Real $n, LengthAbbrevs $abbrev ) {
    return Length.new(v => $n * $abbrev);
}

multi infix:<+>( SI $x , SI $y where .isa($x) )  {
    $x.WHAT.new( v => $x.v + $y.v);
}

say 10[kg] + 1[g];
say 1[km] + 1[m];
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