[perl #131491] Using infix: to set default sometimes ends up as Seq in `where` clause

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# New Ticket Created by  Zoffix Znet 
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The issue I encountered is a case where when I assign a default created using 
infix:<xx> op,
inside the `where` clause, it's not yet failed over, so I got exhausted Seq 
errors:

    <Zoffix__> m: -> +@commands, :@in where {dd $_; 1} = Nil xx 1 { @in.elems 
}([])
    <camelia> rakudo-moar 61ecfd: OUTPUT: «(Nil,).Seq␤»

Looking at it further, however, it appears most of the time it Does The Right 
Thing;
and in fact, seems to be affected by unrelated things, such as whether there's
another parameter before it and whether it got a `+` slurpy on it:

    <Zoffix__> m: -> @commands, :@in where {dd $_; 1} = Nil xx 1 { @in.elems 
}([])
    <camelia> rakudo-moar 61ecfd: OUTPUT: «(Nil,)␤»
    <Zoffix__> m: -> :@in where {dd $_; 1} = Nil xx 1 { @in.elems }()
    <camelia> rakudo-moar 61ecfd: OUTPUT: «(Nil,)␤»