#perl mytest.pl sn 5

# Before anything else is done, @ARGV looks like:
# $ARGV[0] = 'sn'
# $ARGV[1] = '5'

my $based_on = $ARGV[0] ? shift: 'tp';

# Now @ARGV looks like this:
# $ARGV[0] = '5'
# $ARGV[1] = undef

my $top = $ARGV[1] ? shift: 1;

# It's going to return '1' because $ARGV[1] was undef, due to
# the first shift.