>perl -wMstrict -le
"my $s = '1a2b3c';
 ;;
 print qq{no /o};
 for my $i (qw(3 2 1)) {
   print qq{matched '$1'} if $s =~ m{ ($i.) }xms;
   }
 ;;
 print qq{with /o};
 for my $i (qw(3 2 1)) {
   print qq{matched '$1'} if $s =~ m{ ($i.) }xmso;
   }
"
no /o
matched '3c'
matched '2b'
matched '1a'
with /o
matched '3c'
matched '3c'
matched '3c'