for my $letter ('A'..'Z') {
    my $file = "tmp/$letter.txt";
    open my $fh, '>', $file or die "No open > $file: $!";

    say $fh join "\t", 'Gene', map "exp$_", 1..10;

    for my $i (1..2000) {
        say $fh join "\t", $i, map "$letter-$i-exp$_", 1..10;
    }
}