my @output;
my $pk = "SomeString";
foreach my $pool ( @pools ) {
   my @machines = qx/Some command that gets me a list of machines/;
   #Some code here to check for cmd errors, etc. Convert output to an array of hashes (decode_json is involved);
   my $machine_aref = decode_json $machines[0];
   foreach my $row ( @{$machines_aref} ) {
      push @output, $row->{$pk};
   }
}
#At this point I can see the full list of machines, with no issues, or empty lines
@output = uniq @output;
#At this point @output is empty
return \@output;

##</code><code>##

Same loop to generation to generate the arrays. Still seeing all values in the array before this:
   my @final_output;
   foreach my $line ( @output ) {
      next if ( grep m/^$line$/ @final_output );
      push @final_output, $line;
   }   
<\code>
<br>
This time I don't see 0 results, the code just hangs. At first I thought, wow, is it really that inefficient? So I decide I can do better.<br>
<br>
From a little reading about uniq I find out it essentially just slams the array values into a hash and then pulls the keys. I don't care about ordering, so why not just roll that myself?<br>
<br>
<code>
my %output_hash;
my $pk = "SomeString";
foreach my $pool ( @pools ) {
   my @machines = qx/Some command that gets me a list of machines/;
   #Some code here to check for cmd errors, etc. Convert output to an array of hashes (decode_json is involved);
   my $machine_aref = decode_json $machines[0];
   foreach my $row ( @{$machines_aref} ) {
      $output_hash{$row->{$pk}} = 1;
   }
}

my @output_array = keys %output_hash;

return \@output_array;

##</code><code>##

my %output_hash;
my $pk = "SomeString";
foreach my $pool ( @pools ) {
   my @machines = qx/Some command that gets me a list of machines/;
   #Some code here to check for cmd errors, etc. Convert output to an array of hashes (decode_json is involved);
   my $machine_aref = decode_json $machines[0];
   print "$#{machine_aref} lines returned\n";
   foreach my $row ( @{$machines_aref} ) {
            unless ( defined $row->{$pk} ) {
               print "'$pk'\n";
               print Dumper $row;
               print "$pk, $row\n";
               print "Bad return: Some command from above\n";
               print "HERE: " . $row->{"$pk"} . "\n";
               die;
            }
      $output_hash{$row->{$pk}} = 1;
   }
}

my @output_array = keys %output_hash;

return \@output_array;

##</code><code>##

<PoolName>: 4737 lines returned.
'SomeString'
$VAR1 = {
          'SomeString' => '<machinename>.<location>.<something>.com',
          'Count' => 6
        };
SomeString, HASH(0x106d978)
Bad command return: <Some command used to get this data>
Use of uninitialized value in concatenation (.) or string at <script_path> line <Line with print "HERE: " .$row->{"$pk"} . "\n";>.