Change the order of your tests. Note that No Arguments and default are the same thing:

if (@ARGV){ 
    print "Command line args\n";

} elsif (! -t STDIN) {
    print "STDIN\n";

} else {
    print "Default\n";
}
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I don't believe your -t STDIN test will work though. Don't know what the right test is for that at the moment.

Update: Fixed STDIN test per Anon Monk's post.