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

##</code><code>##

my %deps;
while(<STDIN>){
  my @cols = split;
  my $task = shift @cols;

  $deps{$task}->{$_} = undef for @cols;
  # or another way:
  %{$deps{$task}} = map { $_ => undef } @cols;
}