my $uripath = 'http://www.somewhere.com/~s/reports/?C=M;O=D';

# easier to maintian as a partial expression
# added support for more sortorders
# which is probably where you erred
my $rx_dirsort = qr{\?(C=[NMSD];O=[AD])$};

print "hit!\n" if $uripath =~ /$rx_dirsort/;
[download]

hth

Edit: Actually, the most important error you made was using the final '?', which has been pointed out by many here.