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while (<DATA>) {
    chomp;
    my ($head, $seq) = split / /;

while ( <DATA> ) {
    my ($head, $seq ) = split;

my @file_ends = qw{./ .fasta};
...
                open my $fh, q{>>}, join($ko, @file_ends);

my file_end = '.fasta';
...
                open my $fh, '>>', "$ko$file_end" or die "Cannot open 
+'$ko$file_end' because: $!";