Try closing your workbook explicitly.

package excelUtil; 

use Exporter;
use strict;
use Spreadsheet::WriteExcel;

use vars qw(@ISA @EXPORT);

@ISA    = qw(Exporter);
@EXPORT = qw(writeCell close_workbook);

## Excel workbook
my $workbook = Spreadsheet::WriteExcel->new("Sample.xls");

## worksheet
my $worksheet = $workbook->add_worksheet("test");

## Functions
sub writeCell {
    my ($row, $col, $str) = @_;

    print "Writing Cell \n";
    $worksheet->write($row, $col, $str);
}

sub close_workbook {
    print "Closing workbook\n";
    $workbook->close();
}

writeCell(1, 2, "testing, Row-1, Col-2");

1;
[download]

#!/usr/bin/perl

use excelUtil;

writeCell(1, 1, "Testing, Row-1, Col-1");
close_workbook;
[download]

Edit: corrected my cut and paste error for the second block (which was a duplicate of the first), as noted by soonix - many thanks! What you suggest is exactly what I intended.