In the section "Returning Failure," PBP recommends using a bare return to signify failure. There is a two page discussion there, explaining that, at the hands of the caller, the returned undef can easily turn into the one-element list (undef) and evaluate to true.

    sub foo { return undef;}
    sub bar { return;}
    if (my @foos = foo()) {
        print "foo() is so true...\n";  ## surprise!
    }
    else {
        print "foo() is false.\n"
    }
    if (my @bars = bar()) {
        print "bar() is true...\n";
    }
    else {
        print "bar() is false.\n"
    }
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