(my $spec_file     = shift) or die $syntax;
(my $data_in_file  = shift) or die $syntax;
(my $data_out_file = shift) or die $syntax;
# etc...
[download]

Yuk!

die $syntax unless @ARGV == 5;
my ($spec_file, $data_in_file, $data_out_file, $log_file, $max_errors)
+ = @ARGV;
[download]

or

my ($spec_file, $data_in_file, $data_out_file, $log_file, $max_errors)
+ = @ARGV;
die $syntax unless defined $max_errors;
[download]

Do not repeat yourself!