After reading the documentation, I believe this can be done by changing use Getopt::Long; to use Getopt::Long qw(:config require_order);.

require_order

Whether command line arguments are allowed to be mixed with options. Default is disabled unless environment variable POSIXLY_CORRECT has been set, in which case require_order is enabled.

See also permute, which is the opposite of require_order.

Getopt::Long will stop processing flags and such once it sees an argument without a dash.

Great, thank you!

This runs into a problem if the negative number is the first argument, since Getopt::Long then has no way to know that it's not a genuine flag. The solution is the usual GNU-style -- flag, which signals the end of options (so that require_order isn't even necessary, though it'll do no harm—besides being more natural, I think).

In your case, perhaps Getopt::Std will be just fine. This not to say that that other suggestions aren't fine also.

use Getopt::Std;
my %opts;

die "Usage $0 -m or -M #1 #2 #3..." if (!getopts( "mM", \%opts)
    || @ARGV !=0 || keys %opts != 2);
CalculateMean()   if $opts{m}; #@ARGV has just #1 #2 #3 ...now.
CalculateMedian() if $opts{M};
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getopts::Std take a string and a ref to a hash as args. The magic of getopts in its various flavors is that it removes the option and the parameter (if any) from @ARGV.

There is a syntax for this: "mf:" means that "-m" doesn't have an arg, but that "-f" does, like someprogram -m -f filename 1 2 3. In this case the %opts hash will have keys of "m" and "f". The value of $opts{m}=1, the value of $opts{f} = 'filename' and @ARGV = (1,2,3).

You can have things like -a -f filename 1 2 3 4 or -f filename -a 1 2 3 4. Of course if somebody types in: -a -f 1 2 3 4 filename, you have to figure out that "1" isn't a filename. Anyway getopts looks for essentially space separated tokens, -3 like +3. That's the way that a C program would get the args.

I hope this helped.

Sorry there is an extra paren in the code. Don't know how to edit this out yet. corrected now.