BEGIN{} blocks are executed as soon as they are compiled, before the rest of the file gets compiled. So you tried to call the subroutine before it was compiled.

You need to move the sub help {...} into the BEGIN{} block.

I assume the missing sub keyword was just a typo in the example.

Take a look at Getopt::Long. Its pod has examples of that very thing, in a robust and extensible setting. Pod::Usage automates help, extracting the text from your own pod. The two work well together. Very useful!

holli has correctly identified your syntax error. You can avoid the ampersand before the help() call by defining the sub before calling it, or by calling it with empty parens.

Argument handling is typically handled by a module in the Getopt family. The second part of the sentence indicates that you want to check the arguments before using a module though. This is a bit harder since use statements are handle at compile time, not run time. Typically this is handled by using require in lieu of use and running the import manually. Messing with BEGIN blocks as indicated elsewhere may work, but YMMV.

In this case, your code is complaining because you have not defined the help subroutine properly (see answer above for how). Typically, you should get out of the habit of calling subroutines as &sub_name and use sub_name() instead. This is because it affects @_. If none of this makes sense to you, see perldoc perlsub and perldoc perlvar.

sub help {}
[download]

sub help{
   print "Whatever youd like to write\n";
   return;
}

die &help unless scalar(@ARGV) == 1;
[download]

use strict;
use Getopt::Long;

my $var;

sub usage{
   print "Usage: $0 --foo=bar\n";
   return;
}

GetOptions("foo=s" => \$var) and defined($var) or die &usage;

print "You typed: $var\n";
[download]

Unless your modules are doing really crazy things, a reasonable number of modules will load on any modern^* machine in less time than a human can register. In other words, don't worry about it, use Getopt::Long, and be about your business.

*: "Reasonable" as in less than 100. "Modern" as in newer than 1995.