Laurent_R, GotToBTru thanks a lot to both of you for pointing out mistakes in my code. I don't know how could I not notice them before. I have followed your advices to edit the code and also edited few other things and now it works as expected.

#!/usr/bin/env perl
use strict;
use warnings;
use File::Basename;
my $lab_root = dirname $0;

opendir( DH, $lab_root) or die "Cannot open $lab_root: $!\n";
my @kickstarts = grep ( /\.ks$/, readdir(DH));

my @bsname ;
my @predefined;
my $hostname;
for my $kickstart (@kickstarts) {
    open my $fh, $kickstart or die "Cannot open $kickstart: $!";

    while (<$fh>) {
        chomp;
        if ( /(?s)^((?!#).)*--hostname=/ ) {
            my @fields = split /[=\s]/;
            $hostname = $fields[1];
            push @bsname , $hostname;
            push @predefined , $kickstart;
        }
    }
    close $fh;
    next if $kickstart ~~ @predefined;

    $hostname = (split /\./, $kickstart )[0];
    push @bsname, $hostname;
}
[download]

Hi Yakup,

Glad to hear your code is doing what you want. I do have to admit I found the logic a bit hard to follow, but that isn't necessarily a problem. I would however recommend you run the code through enough test cases, just to make sure.

I just wanted to point out a few potential improvements:

• Using FindBin to locate the directory that the Perl script is located in is much more reliable than dirname $0.
• Using glob is easier than opendir+readdir (with the caveat that glob does things a little bit differently, for example it ignores files whose name begin with a dot by default; see File::Glob for all the details); alternatively there are modules like Path::Class or Path::Tiny to help.
• This is just a stylistic thing, but personally I wouldn't make the regex as complex. I might write something like next if /^\s*#/; if (/--hostname=/) ...
• Smart match (~~) has been placed into experimental status and its implementation might change in the future, so in this case something like List::Util's any is better for now. Or, if all you're using @predefined for is to check whether certain strings have been seen before, a hash would be much better.
• The code (split /\./, $kickstart)[0] will only return the first part of $kickstart up to the first dot anywhere in the string, so it will not do what you want (strip the .ks suffix) for strings like "some.file.name.ks" or "/some.path/some.file.ks". I would recommend a regex, or the all-out solution is File::Basename's fileparse.
• Also, I'd recommend the three-argument form of open, and to limit the scope of your variable declarations to where they are needed.

If I implement those suggestions, I get:

#!/usr/bin/env perl
use strict;
use warnings;
use FindBin;
use File::Basename qw/fileparse/;
use List::Util qw/any/;

my $lab_root = $FindBin::Bin;
my @kickstarts = glob "$lab_root/*.ks";

my @bsname;
my @predefined;
for my $kickstart (@kickstarts) {
    open my $fh, '<', $kickstart or die "Cannot open $kickstart: $!";
    while (<$fh>) {
        chomp;
        next if /^\s*#/;
        if (/--hostname=/) {
            my @fields = split /[=\s]/;
            push @bsname, $fields[1];
            push @predefined, $kickstart;
        }
    }
    close $fh;
    next if any {$kickstart eq $_} @predefined;
    my $hostname = fileparse($kickstart, qr/\.ks$/i);
    push @bsname, $hostname;
}
[download]

Note that in this code compared to yours, the filenames now all have their pathnames prefixed. Since I don't know what the rest of your code does, this may or may not be a problem for you. If it is, you could for example use the same fileparse function to get only the filename.

Hope this helps,
-- Hauke D

Hello Hauke, thanks for your comment! I find your suggestions very helpful, especially glob and fileparse functions really make things easier and more flexible.