line 4: yes, your assumption is correct -- $max_so_far is a scalar .. These two lines are equivalent:

my($max_so_far) = shift @_;
my $max_so_far = shift @_;
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You can, however, declare several scalars at the same time, in which case you use the parens:

my ($x, $y, $x) = (1, 2, 3);
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As for line 10, yes, that last line does the same as return $max_so_far; (which would have been more explicit). From perldoc -f perlsub: "The return value of a subroutine is the value of the last expression evaluated."
Trying it with line 10 commented out, $maximum is undef .. i tried w/the max value last in the list, too, so i supsect it's not the last "if", but that the undef comes from the loop termination..