There are two problems with the syntax you used above. First, the || is better written as or, although in this particular case, the effect is more or less the same. A worse problem is that your code will die if the program is given an argument that's false, for instance 0.

I'd write something like:

    my $argument = @ARGV ? shift : do {
        print "Please give an argument: ";
        my $arg = <STDIN>;
        chomp $arg;
        $arg;
    };
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Abigail