You already have some code to scan through your directory and decide whether or not you want a file, so all you have to do is push the file into the array at the end of a similar loop. Something like the following:

    while (my $file = readdir(DIR)) {
        
        #decide whether or not this is a file the user wants
        ...

        push @files, $file;
    }
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Then you can use stiller's loop to iterate through that list, opening each file for processing.

How to get the range, lets say, in this directory I have 30 files, from day 1 to day 30, form this dropdown menu I select from day 5 to day 25, and then do the processing, open all the files between day 5 and day 25, how can I do that on this code?

you could use

my @files;
if (-d $x_file) {
   # $x_file is a directory, not a file
   @files = glob "$x_file/*xml";
}
elsif ($x_file =~ /xml$/ and -e $x_file){
   # $x_file is an xml-file, and we have access to it
   push @files, $x_file;
}
else {
   die("can't make sense out xml-directory or xml-file $x_file");
}
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