I don't see an obvious cause of the error you get in the code you posted, but what you posted is obviously not what you ran when you got the error, so that doesn't mean much.

Maybe the following code does something like what you want and you can incorporate how it does it into your code.

use strict;
use warnings;
use Data::Dumper;

package ClassA;

sub new {
    my $class = shift;
    my ($client) = @_;
    my $self = bless {}, $class;
    if($client eq 'Veda') {
        $self = ClassB->new();
    }
    return $self;
}


sub login {
    print "ClassA login\n";
}

package ClassB;

sub new {
    my $class = shift;
    bless {}, $class;
}

sub login {
    print "ClassB login\n";
}

package main;

print "obj1\n";
my $obj1 = ClassA->new('Veda');
print Dumper($obj1);
$obj1->login();

print "obj2\n";
my $obj2 = ClassA->new('Other');
print Dumper($obj2);
$obj2->login();
[download]

Output is

obj1
$VAR1 = bless( {}, 'ClassB' );
ClassB login
obj2
$VAR1 = bless( {}, 'ClassA' );
ClassA login
[download]