If a regex is defined in string form, then no regex modifiers are associated with it unless they happen to be explicitly embedded within the string using a  (?adlupimsx-imsx) or  (?adluimsx-imsx:pattern) construct (see Extended Patterns in perlre).

c:\@Work\Perl\monks>perl -wMstrict -le
"my $rs1 = '(?:(?-s).*)';
 ;;
 my $rx1 = qr{ ($rs1) (.*) }xms;
 print qq{\$rx1: $rx1};
 ;;
 qq{foo\nBAR} =~ m{ $rx1 }xms;
 print qq{from \$rx1:  \$1 '$1'   \$2 '$2'};
 ;;
 my $rs2 = '(?:.*)';
 ;;
 my $rx2 = qr{ ($rs2) (.*) }xms;
 print qq{\$rx2: $rx2};
 ;;
 qq{wig\nWAG} =~ m{ $rx2 }xms;
 print qq{from \$rx2:  \$1 '$1'  \$2 '$2'};
"
$rx1: (?^msx: ((?:(?-s).*)) (.*) )
from $rx1:  $1 'foo'   $2 '
BAR'
$rx2: (?^msx: ((?:.*)) (.*) )
from $rx2:  $1 'wig
WAG'  $2 ''
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