How to name a constraint

You can try to generate the error in the where clause with the || operator.

my &simp = -> *@a where { (0 <= $_.all <= 255) || die 'not in Range' } { @a <<+>> 10 }
say &simp( 2, 3, 4);
# returns: [12 13 14]

say &simp( 2,3,400 );
#not in Range

What you want is a subset.

subset ByteSizedInt of Int where { 0 <= $_ <= 255 };
my &simp = -> ByteSizedInt *@a { @a <<+>> 10 };