Note, a BEGIN block isn't necessary -- a bare block is fine as long as it is defined prior to the sub calls. Also, a BEGIN block may not always be the appropriate mechanism:

#!/usr/bin/perl -w
use strict;
use Getopt::Std;

my %opt;
getopt('n',\%opt);

BEGIN
{ 
  my $foo = $opt{n};
  sub up { 
    $foo++; 
    sub get {$foo} 
  } 
}

up(); 
up(); 
print get();
[download]

Now, if you comment out the BEGIN line above it will work as one might have expected. Not that I think this might be a common problem, but I thought it might be worthwhile to highlight the difference.