Perl has at least seven eight cats without leaving the core or shelling out. And I'm sure I'm missing more than I identified.

$new = $string_a . $string_b; #              1 cat.
$new = join '', $string_a, $string_b; #      2 cats.
$new = "$string_a$string_b"; #               3 cats.
$new = pack '(A*)*', $string_a, $string_b; # 4 cats.
$new .= $_ for $string_a, $string_b; #       5 cats.
$new = do {
  open my $fh, '>', \my $var;
  print {$fh} $string_a, $string_b;
  close $fh;
  $var;
}; #                                         6 cats.
$new = List::Util::reduce { $a .= $b } $string_a, $string_b; # 7 cats.
$new = sprintf '%s%s', $string_a, $string_b; # 8 cats.
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