Win8 Strawberry 5.8.9.5 (32)  Sat 07/23/2022 16:42:27
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

use Data::Dump qw(dd);

dd 'A: $1', $1;

my $rx = qr/(#\S+).+$1/;
print "B: $rx \n";

'OOPS' =~ /(OOPS)/;
$rx = qr/(#\S+).+$1/;
print "C: $rx \n";
^Z
("A: \$1", undef)
Use of uninitialized value in concatenation (.) or string at - line 8.
B: (?-xism:(#\S+).+)
C: (?-xism:(#\S+).+OOPS)