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%deps = (
  1 => { 2 => undef, 3 => undef },
...
  3 => { 1 => undef, 4 => undef },
  5 => { 2 => undef, 3 => undef },
);

my %deps;
while(<STDIN>){
...
  # or another way:
  %{$deps{$task}} = map { $_ => undef } @cols;
}