hum, I had tested

BEGIN {
     $c = <<USAGE;
What do you mean, too late?
USAGE
     use constant USAGEMSG => $c;
}
[download]

and

BEGIN {
     $c = <<USAGE;
What do you mean, too late?
USAGE
}
use constant USAGEMSG => $c;
[download]

and both gave me the same error. Maybe the file wasn't properly saved for the second test?

Thanks for verifying; it puzzled me.