my $foo = [ [ 4, 1, 6 ], [ 4, 1, 5 ] ];
my $foo_0 = $foo->[0];
my $foo_1 = $foo->[1];

 $foo_0 = [ sort @$foo_0 ];   # $foo_0 is "detached"
@$foo_1 = sort @$foo_1;       # Goes back into $foo->[1]

use Data::Dumper;
print Dumper($foo);