my $long = 'D:/temp/spa ce/ab cd.txt';
my $short = Win32::GetShortPathName($long);
system('start', $short);
[download]

This will also avoid the problem that spaces in the actual filename cannot be handled/escaped in the start call.

It's not necessary to issue a use Win32;, since GetShortPathName is accessible as core function in AS perl on Win.