A memory benchmarking of the two modules supports better memory usage by Spreadsheet::ParseExcel::Stream for the task below on a 1.9M SS, 20 sheets, each having 500 x 26 cells filled:

use strict;
use warnings;
use Memchmark qw(cmpthese);
use Spreadsheet::ParseExcel;
use Spreadsheet::ParseExcel::Stream;

my $xls_file = 'Book1.xls';

cmpthese(
    Spreadsheet_ParseExcel_Stream => sub {
        my $xls = Spreadsheet::ParseExcel::Stream->new($xls_file);

        while ( my $sheet = $xls->sheet() ) {
            my $cellA1 = $sheet->row->[0];
        }
    },
    Spreadsheet_ParseExcel => sub {
        my $parser   = Spreadsheet::ParseExcel->new();
        my $workbook = $parser->parse($xls_file);

        for my $worksheet ( $workbook->worksheets() ) {
            my $cellA1 = $worksheet->get_cell( 0, 0 )->value;
        }
    }
);
[download]

Results:

test: Spreadsheet_ParseExcel, memory used: 199147520 bytes
test: Spreadsheet_ParseExcel_Stream, memory used: 17633280 bytes
[download]

As a side note, Spreadsheet::ParseExcel::Stream is a front end for Spreadsheet::ParseExcel, and its author asserts that its memory management is optimized.