You have circular inheritance - A uses B uses A uses B... Perl resolves this by importing a module only once. In your case, this means that depending on the order of processing, abc() may or may not be exported before def() is defined.

The solution is don't do this. Circular dependence can usually be replaced with simple inheritance with no loss of functionality. In this case, the 'solution' is to simply not have module A use B. More often, the easiest solution is abstracting the shared code into a common module 'C'.

#!/usr/bin/perl 
use lib ".";
use A;
use B; 
def();
######################################## 
A.pm 
package A;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(abc); 
use C;
######################################## 
B.pm 
package B;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(def);
use C; 

sub def{
    print "Inside sub def of package B -> calling sub abc from package
+ C\n";
    abc();
}
######################################## 
C.pm 
package C;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(abc); 

sub abc{
    print "Inside sub abc of package A\n";
}
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