use Data::Dump 'pp'; # a "better" Data::Dumper

my %obs;
while ( <DATA> ) {
    chomp;
    my ( $obs_num, $det, $x, $y ) = split;

    push @{ $obs{$obs_num}{$det} }, { x => $x, y => $y };
}

#$obs{$obs_num}{$det}[$reading]{x}

pp \%obs;

__DATA__
21 DET-2 896.657564735788 678.83860967799
21 DET-3 32.0939023018969 621.656550474314
21 DET-3 42.0741462550974 834.842294892622
21 DET-3 218.814294809857 450.606540154849
21 DET-3 228.88830316475 625.939190221948
21 DET-3 630.472705847461 220.839350101088
21 DET-5 152.988115061449 156.31861287082
88 DET-0 114.871177986263 212.959076023136
88 DET-0 219.421725079137 710.314439572696
88 DET-0 257.837516726887 594.376577764894
88 DET-1 119.630462310966 260.433234269099