When you open a file in Perl, it gets created if it didn't already exist. I don't know what Tk has to do with this, but I assume you're displaying the spreadsheet there, somehow. If you don't want it saved just yet, do something like the following:

use Excel::Writer::XLSX;

#Create a filehandle that just points to a scalar for now.
my $excel_data = "";
open( XLS_TMP_FH, \$excel_data );

#Call the workbook constructor
my $workbook = Excel::Writer::XLSX->new( \*XLS_TMP_FH );

#...

#Chose if you want to save it
if( i_want_to_save_it() ) {
    open( XLS_FILE, ">", "myfile.xlsx" ) or die( "Cannot open file: $!
+" );
    print XLS_FILE join( "", <XLS_TMP_FH> );
    close XLS_FILE;
}
close XLS_TMP_FH;
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I don't know if that will actually work(see signature :-), but it's worth a shot.

my $workbook =