my $string = "one two three four five six seven eight nine ten eleven twelve thirteen";

my( $left, $right );

if( ( $left, $right ) = $string =~ m{\A((?:\S+\s){11})(.+)\z} ) {
    say "Left: ($left)";
    say "Right: ($right)";
}

##</code><code>##

Left: (one two three four five six seven eight nine ten eleven )
Right: (twelve thirteen)

##</code><code>##

m{\A((?:(\S+\s){10}\S+)\s(.+)\z}