abc.pl 
#!/usr/bin/perl 
use lib ".";
use A;
use B; 
def();
######################################## 
A.pm 
package A;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(
abc
); 
use B; 
sub abc{
print "Inside sub abc of package A\n";
}
######################################## 
B.pm 
package B;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(
def
);
use A; 
sub def{
print "Inside sub def of package B -> calling sub abc from package A\n";
abc();
}