#!perl

use strict;
my @array=();
while (<DATA>){
  chomp;
  my ($sentence, $attribute, $value) = split;
  # use [] not () to push array refs
  push @array,[$sentence,$attribute,$value];
}

my $selected_order=1; #order according to $attribute
# retain the result of sort in new array 
my @sorted_array = sort { $a->[$selected_order] cmp $b->[$selected_order] } @array;
# or use existing
# @array = sort { $a->[$selected_order] cmp $b->[$selected_order] } @array;

# get values from sorted array 
# by dereferencing
for my $record (@sorted_array){
  print "@$record\n"
};

__DATA__
sentence1 attribute3 value2
sentence2 attribute2 value1
sentence3 attribute1 value3