Assuming your FASTA records are single-line:

$ awk -vPRIMER="ACTG" '{if ($0~/^>/) { print $0; } else { print PRIMER$0; }}' in.fa > out.fa

If not, you can pre-process your FASTA file to make it single-line.

The first ever perl script I wrote does this! Adds a different barcode based on a common fasta header identifier, which I think that's what you want?

All you do is add_barcode.pl my_file.fasta then it'll ask you for the shared ID in the header, then ask for what the barcode you want to add is. Not the best code in hindsight, but does the job!

#!/usr/bin/perl    
use 5.010;

print "USAGE: add_barcode.pl my_file.fasta\n"

chomp($file = $ARGV[0]);
$file =~ s/\..*//;

print "what is the match (case-insensitive):\n";
chomp($site = <STDIN>);

print "what is the sites barcode:\n";
chomp($barcode = <STDIN>);


chomp(@lines = <>);

open MODIFIED, ">$file" . "_barc.fas";
select MODIFIED;
foreach (@lines){
        if (/$site/i) {
        print "$_\n$barcode";
        $count += 1;
        }else{
        print "$_\n";
        }
}
close MODIFIED;
select STDOUT;


print "modified $site $count times\n";