Per Abigail-II's trail of thought with context sensitivity added:

sub filter (&@) {
    (wantarray ?
        sub { @_ } :
        sub { join '', @_ }
    )->(
        map { local $_ = $_; $_[0]->(); $_ }  @_[1 .. $#_]
    );
}
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However, I really find this silly, as you can simply do the following:

sub filter (&@) {
    my ($func, @out) = @_;
    $func->() for @out;
    wantarray ? @out : join '', @out;
}
[download]

Makeshifts last the longest.