use strict;
use warnings;

my @matrix = (
  [48, 219, 11021],
  [72, 190, 11006],
  [203, 177, 11005],
  [301, 186, 11013],
  [309, 119, 11015],
  [309, 216, 11017],
  [309, 147, 11000],
  [343, 180, 10990],
  [346, 179, 10989],
  [451, 258, 11008],
  [838, 162, 11014]
);

# e.g. filter out all sub-arrays with x <= 309:
@matrix = grep { $_->[0] <= 309 } @matrix;

local $, = " ";
local $\ = "\n";
print @$_ for @matrix;

__END__

Output is:

48 219 11021
72 190 11006
203 177 11005
301 186 11013
309 119 11015
309 216 11017
309 147 11000