I wrote a program to do exactly this. This is very bad code, as it was mostly a one-off for a batch process, and after I got it working, I moved on. I was also not very good with Perl at the time.

#!/usr/bin/perl

use strict;
use warnings;

use Spreadsheet::ParseExcel::Simple;
use Text::CSV_XS;
use IO::File;
use File::Spec;

for (@ARGV) {
    for (glob $_) {
        next unless m/\.xls$/i;
        next unless -r $_;
        dump_books($_);
    }
}

sub dump_books {
    my ($vol, $path, $file) = File::Spec->splitpath(shift);
    my $eBook = Spreadsheet::ParseExcel::Simple->read(File::Spec->catp
+ath($vol,$path,$file));
    my @sheet = $eBook->sheets;
    for (0..@sheet-1) {
        next unless $sheet[$_]->has_data();
        my $sfn = $file;
        $sfn =~ s?\.xls$??i;
        $sfn.= ((@sheet > 1) ? sprintf(".%02i",$_) : "").'.csv';
        
        my $io = IO::File->new();
        $io->open('<'.$sfn) or do {
            warn "Unable to write to $sfn";
            next;
        };
        
        my $csv = Text::CSV->new;
        
        while ($sheet[$_]->has_data) { 
            my @row = $sheet[$_]->next_row;
            $csv->print($io, \@row);
        }
        
        $io->close;
    }
}
[download]

This only really works with ActiveState Perl, but could be tweaked by altering the parameters to the Text::CSV_XS new() call. There's lots of error handling and process improvement that could go on here, and it will fail miserably on complex XLS books.

On the upside, each sheet in an Excel book will end up in its own CSV file. Hopefully this will give you a place to start.