This works fine for me:

use strict;
use warnings 'all';

my $found = 0;

for my $in_path (@INC){
    next if ! -e $in_path."\\Win32API\\File.pm";
    print "found module in $in_path ...\n";
    $found = 1; 
    last;
}

if ($found == 1){
    use Win32API::File;
    my @drv = map{s/\\$//;lc}  
               Win32API::File::getLogicalDrives();
    print @drv;
}
[download]

Giving these results:

found module in C:/Perl/site/lib ...
a:c:d:e:p:u:
[download]

SMiTZ

Update: All though this works OK, its pointless putting use into a conditional block as it gets loaded at compile time. Testing for its existance after perl has attempted to load it seems a bit superfluous. Ooops, thanks jmcnamara.