$main->bind("<Control-o>", \&Open);
##</code><code>##

sub Open {
    my $csv_path = $main->getOpenFile( -filetypes => [
        [ 'CSV files', '.csv' ],
        [ 'All Files', '*'    ],
    ] );
    return if not defined $csv_path;

    open my $csv_fh, '<', $csv_path
      or die "Cannot open '$csv_path': $!";

    my $out_fh;
    my $last_filename = '';
    while (<$csv_fh>) {
        my ($f) = /^([^,]+),/;
        my $filename = "$f.txt";
        if ( $filename ne $last_filename ) {
            open $out_fh, '>', $filename
              or die "cannot open $filename: $!";
        }
        print {$out_fh} $_;
        $last_filename = $filename;
    }
}