What plus means in method declaration in perl6?

Question

What does plus mean in method declarations in Perl6?

Here is an example from spec

submethod BUILD (+$tail, +@legs, *%extraargs) {
    $.tail = $tail;
    @:legs = @legs; 
}

Answer 1

2019 Update See the section Variadic positionals destructuring; +@foo and *@foo in my answer to the SO question "variable number of arguments to function/subroutine".

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In 2015 Larry Wall introduced the + parameter prefix, one of four parameter prefixes (*, **, +, |) that signify slurpy (variadic) parameters. He added it to the Rakudo compiler, added some tests, gave a brief informal description of it on the irc channel, and added a section on it to the relevant language design doc.

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The example quoted in the original question is taken from an archive of an informal document written and frozen in time over a decade ago. At that time a + parameter prefix signified a named parameter as contrasted with a positional one. Nowadays we use : for that, thus:

submethod BUILD (:$tail, :@legs, *%extraargs) {
    $.tail = $tail;
    @.legs = @legs;
}

Answer 2

Your "spec" links goes to a historical document, and the syntax has long gone from Perl 6. I'm not sure what it used to do, maybe "at least one argument", in analogy to the + quantifier in regexes.

For an up-to-date specification, please read http://perlcabal.org/syn/S06.html which contains all the information on signatures and subroutines.