use strict;
my $id = 24601;

sub foo {
   # .. lotsa code
   print "ID number is", $id, "\n";
   # .. more code
}

sub bar {
   if ($id = 3) { # Oops-- I've accidentally set $id to 3
      # do something subtle
   }
}

##</code><code>##

BEGIN {
my $id = 24601;

sub get_id {
   return $id;
}

sub set_id {
   my ($new_val) = @_;
   $id = $new_val;
}

}

sub foo {
   # .. lotsa code
   print "ID number is ", get_id(), "\n";
   # .. still more code
}