It sort of depends on what your string looks like, but here is one solution:

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)";
}
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Output:

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

If you want to gobble that eleventh space, this will do it:

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