but not sure if that really works...

Then just test it for yourself:

use strict;
use warnings;

my @a0;
my @a1 = 1;
my @a2 = 0..5;

check(@a0);
check(@a1);
check(@a2);

sub check {
    if (@_ == 1) {
    #if ((scalar @_) == 1) {
        print "exactly 1\n";
    }
    else {
        print "diff from 1\n";
    }
}

__END__
diff from 1
exactly 1
diff from 1
[download]

Your original method seems to work pretty well.

You could make it more verbose by using scalar. You decide which is simpler for you to understand.