I would like to reload a pre-index file in perl which has been previously indexed as following:

./first_script.pl hg19.fa

use Bio::DB::Fasta;

my $db = Bio::DB::Fasta->new($file_fasta);
..

in an another script I would like to re-load the existing indexed file.

./script.pl hg19.fa.index input

#!/usr/bin/perl

use warnings;
use strict;

use Bio::DB::Fasta;

my $fasta_index_file = $ARGV[0];
my $input_file = $ARGV[1];

open(FH,"<$input_file");

my $db->index_file($fasta_index_file);

In the description it says

 Title   : index_file
Usage   : $db->index_file($filename)
Function: (re)loads a sequence file and indexes sequences offsets in the file
Returns : seq offsets in the file
Args    : filename,
           boolean to force reloading a file

but it returns the following error:

Can't call method "index_file" on an undefined value at script.pl line