In both branches of your if statement, your string contains meta-characters, but on unix the shell expands environment variables as each statement is executed, while on windows they are only expanded once when the line is read. On my windows server, this is what happens when I run your command:

R:\>set test=hello && echo %test%
%test%
[download]

R:\>set test=hello && cmd /c "echo %test%"
hello
[download]

This should work:

use strict;
use warnings;

if($^O =~ /mswin32/i){
    system(qq!set test=hello && cmd /c "echo %test%"!);
}else{
    system("test=hello; export test; echo \$test");
}
[download]