[perl #76682] [BUG] Doing .substr outside of Str yields a Mu in Rakudo

[Carl Mäsak]

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<masak> rakudo: say "foo".substr(4).WHAT
<p6eval> rakudo 937177: OUTPUT«Mu()␤»
* masak submits rakudobug

Trying to do further string operations on the Mu object (such as
string concatenation) turns out not to work too well.

No applicable candidates found to dispatch to for 'infix:<~>'.
Available candidates are:
:(Any $x = { ... })
:(Any $a, Any $b)

I'd expect it to return "".