I tried to find out on Net and found out there are modules present public, private, but if I want to implement without using those modules, how can I implement this feature?

This is not a very easy problem to solve in perl. Here's one possible approach (though not foolproof):

use strict;

package BOB;
use Carp;
use base 'IO::File';
use overload();

sub _pRiVaTish {
    my $self = shift;
    my ($package, $filename, $line) = caller;
    carp "YOU MUST NOT CALL THIS METHOD YOU, YOU AIN'T BOB!!!"
        unless $filename eq __FILE__
        and
        0 == index overload::StrVal($self), __PACKAGE__,0;
    return 'beep';
}

package BIB;
use vars '@ISA';
@ISA='BOB';

1;

package main;

my $bib = BIB->new();

$bib->_pRiVaTish();
__END__
YOU MUST NOT CALL THIS METHOD YOU, YOU AIN'T BOB!!! at private.methods
+.pl line 28
[download]

• perldoc -f caller
• perldoc -f index
• perldoc Carp
• perldoc overload
• perldoc vars