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sub filter (&@) {
    (wantarray ?
        sub { @_ } :
...
        map { local $_ = $_; $_[0]->(); $_ }  @_[1 .. $#_]
    );
}

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