A stringized regex object is wrapped in a non-capturing grouping that preserves the regex modifiers in effect when the regex was compiled. This allows a regex object to be unaffected by the modifiers in effect in any enclosing regex 'scope' during interpolation within another regex or regex object. Consider:

c:\@Work\Perl>perl -wMstrict -le
"my $rx = qr/(?^xms:^data_123$)/;
 print $rx;
 ;;
 my $ry = qr/^data_123$/xms;
 print $ry;
 ;;
 my $rz = qr{ T $rx U $ry V }xms;
 print $rz;
"
(?^:(?^xms:^data_123$))
(?^msx:^data_123$)
(?^msx: T (?^:(?^xms:^data_123$)) U (?^msx:^data_123$) V )
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See also the discussion of  "(?^aluimsx:pattern)" in the Extended Patterns section of perlre.