You create dthe API object but you need to return a result from a call to that object. Note I do not have the dll in question so I can not test but I would think your code should look like this:

use Win32::API;

my $filename="C:/Windows/ieuninst.exe";
my $Api = new Win32::API("fileversion.dll", "GetFileVersionTxt", P, P)
+;

# Set some memory aside for the returned value
my $output = "\x00" x 1024;

# Call the function in the dll
$Api->Call($filename,$output);

# Remove the unused padding in the string

my $final = ($output =~ /(.*?)\x00/);
print "\output - $final";
[download]

This is from an example in Chapter 9 of Win32 API from Win32 Perl Programming by David Roth modified to match your criteria. I would like to play with dll to test this as I think it would need some tweaking to run. I also suggest reading the pod for Win32::API as they have some excellent examples of getting data in and out of the functions loaded.