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;