my $DATA_X;
my $DATA_Y;
while (<>) {
  my ($obs, $det, $x, $y) = split(' ', $_);
  push(@{$DATA_X->{$obs}->{$det}}, $x);
  push(@{$DATA_Y->{$obs}->{$det}}, $y);
}

##</code><code>##

my $DATA;
while (<>) {
  my ($obs, $det, $x, $y) = split(' ', $_);
  push(@{$DATA->{$obs}->{$det}}, {x => $x, y => $y});
}

##</code><code>##

print $DATA->{21}->{'DET-2'}->[0]->{x}; # prints 896.657564735788
print $DATA->{47}->{'DET-7'}->[1]->{y}; # prints 519.649226713148