From @skids

$ perl6 -e 'sub f ($op) { "&infix​:<$op>"; }; f("+").perl.say' # This is OK
"\&infix​:<+>"

$ perl6 -e 'sub f ($op) { "&infix​:«$op»"; }; f("+").perl.say' # This warning should not happen since there is no interpolation of &thing without () after it.
Use of uninitialized value of type Mu in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if needed. in code at -e line 1
"\&infix​:«+»"

$ perl6 -e 'sub f ($op) { "&infix​:«$op»(1,2)"; }; f("+").perl.say' # Whether the warning should happen here is beyond me, the error is correct because an EVAL would be required to dynamically use :infix«...»
Use of uninitialized value of type Mu in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things, if needed. in code at -e line 1
===SORRY!=== Error while compiling -e
Undeclared routine​:
  infix​:<> used at line 1. Did you mean 'infix​:<->', 'infix​:<∖>', 'infix​:<o>', 'infix​:<⊎>'?

autarch++'s original problem leading up to this diagnosis​:
http://irclog.perlgeek.de/perl6/2016-01-16#i_11896249