c:\@Work\Perl>perl -wMstrict -le
"print $];
 ;;
 my $rxs = '(?i) (?: a|b)';
 my $rx  = qr/$rxs/xms;
 print $rx;
"
5.008009
(?msx-i:(?i) (?: a|b))

c:\@Work\Perl\monks\>perl -wMstrict -le
"print $];
 ;;
 my $rxs = '(?i) (?: a|b)';
 my $rx  = qr/$rxs/xms;
 print $rx;
"
5.014004
(?^msx:(?i) (?: a|b))