Perl distinguishes between a capture that wasn't encountered in the match and a capture that matched zero characters.

use Data::Dumper qw( Dumper );

'' =~ /
   ^
   (
      (?:
         (
            bb
         )        # $2
      )?
   )              # $1
   $
/x;

local $Data::Dumper::Useqq  = 1;
local $Data::Dumper::Terse  = 1;
local $Data::Dumper::Indent = 0;

print('$1 = ', Dumper($1), "\n");
print('$2 = ', Dumper($2), "\n");
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$1 = ""
$2 = undef
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