package A;
use B;
require 'standard.pl';
sub new {
#  [snip]
  bless ($self, $class);
  do_something(); # subroutine in standard.pl
  return $self;
}

##</code><code>##

package B;
require 'standard.pl';
sub new {
#  [snip]
  bless ($self, $class);
  do_something(); # subroutine in standard.pl
  return $self;
}

##</code><code>##

# standard.pl
# [snip]
sub do_something {
  print "Howdy.\n";
}
1;

##</code><code>##

Undefined subroutine A::do_something called at A.pm line X.

##</code><code>##

  do_something();

##</code><code>##

  B::do_something();