If I understand correctly, you want the object in the client code to become a different object when a particular method is called on it, correct? If this is want you want you can simply assign the new object to $_[0] in the method. See below:

#File Foo.pm
package Foo;

sub change_var
{
  $_[0] = 'Bar';
}

# File main.pl
use Foo;

my $var = 'Foo';
print "$var\n"; # prints "Foo"

Foo::change_var($var);

print "$var\n"; # prints "Bar"
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In Perl the @_ array actually contains the arguments passed to the subroutine. Modifing @_ directly modifies the variable in the clients namespace. Does that make sense?