From zefram@fysh.org

&prefix​:<+\\\\+>
===SORRY!=== Error while compiling​:
Undeclared routine​:
  prefix​:<+\\+> used at line 1. Did you mean 'prefix​:<++>', 'prefix​:<~>', 'prefix​:<temp>'?

Observe that what I input as <+\\\\+> has been echoed back in the error
message as <+\\+>. Those are not equivalent subscripts, so that error
message is erroneous.

Aside from simply applying the quoting rules to see what the subscripts
mean, one can see that they're different by inputting <+\\+> in the same
context for comparison​:

&prefix​:<+\\+>
===SORRY!=== Error while compiling​:
Undeclared routine​:
  prefix​:<+\+> used at line 1. Did you mean 'prefix​:<++>', 'prefix​:<+>', 'prefix​:<+^>'?

Note that it's echoed differently and gets a different list of suggested
typo resolutions. This time it's actually been echoed back correctly​:
<+\+> is equivalent to <+\\+>.

-zefram

From @zoffixznet

The issue is the message shows literal name of the stuff inside the brackets. Same applies to having a > in it​:

$ perl6 -e '&prefix​:«+\\\>\+»'
===SORRY!=== Error while compiling -e
Undeclared routine​:
  prefix​:<+\\>+> used at line 1. Did you mean 'prefix​:<++>', 'prefix​:<~>', 'prefix​:<temp>'?