my $lineregex = qr{}ms;
$input =~ s{
( # $1 is first 3 lines
  $line   $line   $line
)
( # $2 is second 3 lines
  $line   $line   $line
)
( # $3 is third 3 lines
  $line   $line   $line
)

}{$2$3$1}gx;