The following should get you started. Could be the regexes need a bit of tweaking. Note the use of subs that nicely separate the task into distinct problems and allow easy code reuse.

Your post is nearly unreadable. Dont use </br>. Use <br> or <br/>! I wonder where that meme comes from.

my $desc = get_bug_description(4, 'first.csv', 'second.csv');

sub get_bug_description
{
    my ($id, $key_file, $desc_file) = @_;
    
    my $cve = get_bug_cve($id, $key_file);
    
    open my $file, '<', $desc_file;
    
    while (<$file>)
    {
        return $1
            if /"$cve","([^"]+)/;
    }
    
    close $file;
    
    return;
}

sub get_bug_cve
{
    my ($id, $key_file) = @_;
    
    open my $file, '<', $key_file;
    
    while (<$file>)
    {
        return $1
            if /$id,(.+)/;
    }
    
    close $file;
    
    return;
}
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