use strict;
use warnings;
use autodie;
use Text::CSV_XS 1.18;

print_Filenames_directory ();


sub print_Filenames_directory
{
    my $csv = Text::CSV_XS->new ({ binary => 1, auto_diag => 1 });

    foreach my $file (glob "*.csv") {
        open my $fh, "<", $file or die $!;
                                
        my @name    = split m/_/ => $file;
        my $grpid   = $name[0];
        my $grpname = substr $name[1], 0, -4;
        
        my @headers = ("Group ID", "Group Name", @{$csv->getline ($fh)});
        my @rows;
    
        while (my $row = $csv->getline ($fh)) {
            # update: forgot to remove the two unwanted splice lines
            push @rows, [ $grpid, $grpname, @$row ];
            }
        close $fh;

        open $fh, ">", $file or die $!;
        $csv->say ($fh, \@headers);
        $csv->say ($fh, $_) for @rows;
        close $fh;
        }
    }