By extract do you mean remove? If so, you want splice. See if the following does what you want:

#!/usr/bin/env perl

use 5.010;

use strict;
use warnings;
use YAML;

my @x = ( [ [ qw{ 00 01 } ], [ qw{ 10 11 } ] ] );

say 'Original @x';
print Dump \@x;

splice @{ $x[0] }, 0, 1;

say 'After removing $x[0]';
print Dump \@x;

This prints

Original @x
---
-
  -
    - 00
    - 01
  -
    - 10
    - 11
After removing $x[0]
---
-
  -
    - 10
    - 11