[btilly]

But because Perl is parsed as it is executed, incorrect code raises syntax error only when it is reached by execution.

Sorry, but you're dead wrong. Perl is not parsed as it is executed, which can be verified easily by writing a program with a syntax error at the end, and seeing that it doesn't run code at the top. Try it with the following program.

    print "Hello, world\n";
    This line raises a syntax error before the previous line tries to execute.

What is going on is that BEGIN blocks are special, they are executed as soon as they are parsed. With them we can interleave parsing and execution.

In this case we're assigning to a symbol. And then the parsing of the final line is dependent on whether or not that symbol has a prototype. See https://perldoc.perl.org/perlsub#Prototypes for what Perl prototypes are, and to see why they would affect parsing.