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;