Hi again. First, no way you should be producing your output like that. That kind of manual jiggery-pokery is fraught with pitfalls and error vectors. You need to build up a data structure known as an array of arrays representing your output, and then convert it to the delimited file using Text::CSV or Text::CSV_XS.

As to your problem: you are building a lookup table from the wrong structure. If what you want to do is check all fields in @csvfieldlist to see which are present in @fields, and record a null value for those that are not, you need to make the lookup table from @fields and loop through the larger list.

use Text::CSV_XS;

my @rows;
for my $line ( @something ) {
    if ( $line =~ /NRG\ location/) {
        my @row;
        
        my @fields;
        foreach my $bline (@blocklines) {
            my ($k, $v) = split '=', $bline;
            push @fields, $k;
        
        }
        
        my %lookup = map { $_ => 1 } @fields;

        for my $field ( @csvfieldlist ) {
            if ( exists $lookup{ $field } ) {
                push @row, 'yes';
            } else {
                push @row, undef;
            }
        }

        push @rows, \@row;
    }
}

csv (in => \@rows, out => "file.csv", sep_char=> ";");
[download]

Hope this helps!