my $data = read_file($filename);
# or maybe the reader needs to know the file format too?
my $data = read_file($filename, $fileformat);

##</code><code>##

use 5.010;
use Test tests => 2;

use YourModule qw/read_file/;

my $data = read_file('t/data/ExampleData');

# now you have to ask yourself, what kind of data 
# will you need for processing the spreadsheets?

# I know nothing about your application, so
# I'm just making up some stuff here:
is join(',', $data->headings), 'time,pressure,temperature', 
    'can get column headings from $data';
is $data->temperature( time => "0.01"), 273.018,
    'can retrieve temperature by time';

##</code><code>##

read_process_write(
    inputfiles => [ 't/data/TestInput1', 't/data/TestInput2'],
    merger     => sub { ... },
    outputfile => 't/data/temp/TestResult',
);