So running the below with the pathological case ftp://ftp.ncbi.nlm.nih.gov/genomes/Homo_sapiens/ARCHIVE/BUILD.37.3/CHR_01/hs_ref_GRCh37.p5_chr1.fa.gz

build_chromosome_files.p6

#!/usr/bin/env perl6
use Compress::Zlib;
use IO::String;

my $DATA_DIR = %*ENV<DATA_DIR> // '/Users/matt/data/reference/GRCh37/sequence/dna';

for dir($DATA_DIR).grep(/chr\d+\.fa\.gz/) -> $file {
    my $out_file = $file.IO.extension('txt', :2parts).open(:w);
    my $chromosome = IO::String.new(buffer=>gzslurp($file));
    for $chromosome.lines -> $line {
        FIRST { next }
        chomp $line;
        $out_file.print($line);
    }
}