So, it sounds to me like you have a 'son' number, and you want to find the 'father' of the son, and the father of the father (the sons grandfather you could say).

The code you have will work in your second case, if you just put the variable instead of the number, but there are several more elegant methods you could use, this is how I would probably do it...

sub father_of {
    my $find = shift;

    my $data_file = "reff.data";
    open(DAT, $data_file) || die "Could not open file: $!";
    foreach(<DAT>) {
        chomp;
        my($son,$father) = split;
        if($son == $find) { return $father; }
    }
    close(DAT);
}

my $son = 1045316419;
my $father = father_of($son);
my $grandfather = father_of($father);

print "The son is $son\n";
print "The father of $son is $father\n";
print "The father of $father is $grandfather\n";
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This has a couple of advantages, you have reusable code rather than duplicating the search code every time, and it doesn't read the entire file into memory,

If you did want to read the whole file into memory, storing it as a has makes it easier to use:

open(DAT,$data_file) || die "Couldn't open file":
foreach(<DAT>) {
    chomp;
    my($son,$father) = split;
    $fathers{$son} = $father;
}
close(DAT);

my $son = 1045316419;
my $father = $fathers{$son};
my $grandfather = $fathers{$father};
[download]

(See perldata for more info on hashes)