Rather than using a defined vs undefined value for the op variable, here's an example of that approach using  'i' vs either  '#' or  '' (empty string).

>perl -wMstrict -le
"my $s = 'ASdf';
 ;;
 for my $op ('i', '#', '') {
   printf qq{for op eq '$op': };
   print 'asdf' =~ m{ (?$op) \Q$s\E }xms ? '' : 'NO ', 'match';
   }
"
for op eq 'i': match
for op eq '#': NO match
for op eq '': NO match
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Update:  '-i' also works for turning off (or not turning on) case-insensitive matching.