How do I use a 2 dimensional array for accessing data in a file using Perl?

I am new to Perl coding & am facing a situation. Here is the problem statement:

I have a file which contains some data (only numbers) in matrix form like

1 2 3 4 5 6 .......
7 9 4 6 7 8 .......
...................
...................

I have another file which contains 2 rows of data (some coordinates) like

30 50
04 09
80 90
.. ..
.. ..

These are the coordinates of data situated in the first file, I want to extract that data from the first file & paste in the 3rd row in the second file, for example in the first case I have to search the element in 30th row & 50th column in the first file & paste it in the 1st row 3rd column of the second file.

Solution

This isn't hard.

First, you will have to parse the value matrix. If the input is in the filehandle $fh, then you could do:

my @data;
while(<$fh>) {
  chomp;              # remove newline
  my @row = split;    # split the current line on whitespace
  push @data, \@row;  # put a reference to this row into @data.
}

Arrays can only contain scalars, so we have to put a reference of the row into our data matrix.

The above could be written as

my @data = map { chomp; [split] } <$fh>;

instead; the [...] produces an anonymous arrayref, and map transforms a list by applying the action in the block.

The data can be accessed like $data[$y][$x]. If you want to swap the indices, this becomes akward in Perl, but isn't impossible.

To access the data at the coordinates from the second file (I'll assume the filehandle is $fh2), we just split those lines and look up the element:

while (<$fh2>) {
  chomp;
  my ($x, $y) = split;
  say "at x=$x, y=$y there is $data[$y][$x]";
}

Be sure to use strict; use warnings; at the top of each script to catch errors. say needs perl 5.10 or later.