So, I've read over explanations on making a hash of arrays and attempted to adopt the methods into a program to take a simple datafile and turn it into a hash of arrays. The file has the format

1,2
1,3
1,4
2,3
2,5
2,5
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And I want to make a hash of arrays where the first column is the key and the second column is the data. I've done it like so:

  local %DATAHASH;
  open (DATA, 'foo.csv');
  while (<DATA>){
    chomp;
      my @datarow=split(/,/, $_);
      push(@{$DATAHASH{$datarow[0]}}, ($datarow[1]));
  }
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And yet, whenever I try and print out the contents of DATAHASH, either after the while loop or in its midst using Dumper($DATAHASH}, I get a $VAR1 = undef Any advice on remedying this situation? And looking further down the line, for something that will have two seperate keys (e.g. 1,2,3 is a line from the file) I'm using push( @{$DATAHASH{$datarow[0]}->{$datarow[1]}}, ($datarow[2]) );, which I'm assuming I'll just modify in the same way.