Great, it seems to do the trick... Is it correct that I get all positions in the HoA that you create? 
while(<>)
    %HoA=();
    next unless /^ATOM\b/;
    @split_atom_line=split(/\s+/, $_);
    $chain=$split_atom_line[4];
    $position=$split_atom_line[5];
    $Zcoordinate=$split_atom_line[8];

    if (not exists $HoA{$chain} or $position < $HoA{$chain}[0])
    {
        $HoA{$chain} = [$position, $Zcoordinate];
    }
    
    
    for $key ( keys %HoA ) {
    print "$key: $HoA{$key}[0]\n"; }
}

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In reply to Re^2: How to select specific lines from a file by Anonymous Monk
in thread How to select specific lines from a file by Anonymous Monk

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