$ perl -Mstrict -Mwarnings -e '
    my @fasta = (">dddd\n", "abcdef\n", ">eeee\n", "bcdef\n");

    for (@fasta) {
        unless (/^>/) {
            s/^/ACGAGTGCGT/;
        }

        print;
    }
'
>dddd
ACGAGTGCGTabcdef
>eeee
ACGAGTGCGTbcdef