...if an AUTOLOAD subroutine is defined in the package or
packages used to locate the original subroutine, then that
AUTOLOAD subroutine is called with the arguments that would
have been passed to the original subroutine. The fully qualified
name of the original subroutine magically appears in the
global $AUTOLOAD variable of the same package as the
AUTOLOAD routine

##</code><code>##

# the following code throws a compile time error
package TestPackage;

use strict;

sub AUTOLOAD {
    print $AUTOLOAD, " is not available\n";
}

# this code is fine
package TestPackage;

use strict;
our $AUTOLOAD;    # 'use vars qw( $AUTOLOAD )' works too

sub AUTOLOAD {
    print $AUTOLOAD, " is not available\n";
}