Personally I'd make the intent a little more obvious:

warn "wrong argument"
  if ref $arg{x}
  || !($arg{x} ~~ [qw/a b c/]);

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perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: Smart matching for argument constraints by tobyink
in thread Smart matching for argument constraints by LanX

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