So you have a multi-fasta file like this

GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC   
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 
GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC   
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 
GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC   
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC 

and want to add FASTA description to all of your sequences like this

seq_1
GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_2
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_3
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_4
GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_5
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_6
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_7
GGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_8
AGCTGCTGTAGGGTATGGCGAGCCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC
seq_9
TGGATAATGGACGAGTACCGCCTGGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATCGGCGGATGTAGCCACGTGGATC

How to save perl script

Easiest Way To Install Perl on Windows

So you can use two simple PERL scripts given below to add FASTA description to all sequence of your multi-fasta file
Script1

#!/usr/bin/perl                                                             
# downloaded from www.bioinformatics-made-simple.com

open (INPUT, $ARGV[0]) or die $!;                                           
open (OUTPUT, ">Output.txt");                                                

while (<INPUT>){                                                            
    chomp;                                                                  
    ($seq) = split("\t");                                                   
    print ">seq_$.\n$seq\n";                                                
}                                                                           

close (OUTPUT);                                                             
close (INPUT);

Uses

script-name.pl input.txt >result.txt

Script2

#!/usr/bin/perl -w
# downloaded from www.bioinformatics-made-simple.com

use strict;
use warnings;

    while (<>) {
        print "seq_$.\n" . (/(\S+)/)[0] . "\n";
    }

Uses

script-name.pl input.txt >result.txt