The problem is, as you appear to have noticed, down to the order of calling and declaring the function.

If the declaration precedes any call, perl already knows that print_argu arg,...; is a call to a sub, whereas if the order is reversed i.e. the call precedes the declaration, perl needs a hint as to what sort of symbol print_argu represents. In this case, the hint can be either...

• The addition of the parens in the call or...
• The declaration of the sub prototype (sub print_argu;) before the call

i.e. either of the following will work

print_argu("Hello function!\n");
sub print_argu{
    print "@_\n";
}
[download]

or

sub print_argu;
print_argu "Hello function!\n";
sub print_argu{
    print "@_\n";
}
[download]

That being said, it is recommended, in PBP, that the parens are omitted only for calls to builtins.