use strict;

package A;
    require Exporter;
    our @ISA        = qw/ Exporter /;
    our @EXPORT     = qw/ SayBye /;
    our $language   = "US-English";
    $A::first       = "Alpha";
    sub SayHello    {"hello from package A\n"};
    sub SayBye      {"Goodbye.\n"}

package B;
    require Exporter;
    our @ISA        = qw/ Exporter A /;
    our @EXPORT     = qw/ SayBye /;
    $B::first       = "Bravo";
    sub SayHello    {"hello from package B\n"};
    sub SayByeBye   {SayBye().SayBye();}

package main;
    ### begin_: get stuff from A
    print("$A::first\n");
    print A::SayHello();
    print("$language\n");
    print A::SayBye();
    print "\n---------------------\n";

    ### begin_: get stuff from B
    print("$B::first\n");
    print B::SayHello();
    print("$language\n");
    print B::SayBye(); ### <-- ERROR Undefined subroutine &B::SayBye
    print B::SayByeBye();
    print "\n---------------------\n";