file mod1.pm:
  package mod1;
  use mod2;
  sub import {
    print "mod1 imported from " . (caller)[1] . "\n"
  }
  1;

file mod2.pm:
  package mod2;
  use mod1;
  sub import {
    print "mod2 imported from " . (caller)[1] . "\n"
  }
  1;

file use.pl:
  #!/usr/bin/perl
  use lib '.';
  use mod1;
  use mod2;

output:
  $ perl use.pl
  mod2 imported from mod1.pm
  mod1 imported from use.pl
  mod2 imported from use.pl
  $