without changing the old matrix which is passed as an argument to this function.

There's no such thing as a 2d array in Perl. There are arrays whose elements are references to other arrays. That's what my (@graph) = @_; doesn't make a copy of the "2d array". Search for "deep copy"

Other bugs:

• Start by using use strict; and use warnings;. They will identify at least two errors.

• eq and ne are string comparison operators, but you're using them to compare numbers. Use == and != to compare numbers.

Other tips:

• my $i; for ($i = 0; $i < $graph_size; $i++) {
  [download]

  simplies to

  for (my $i = 0; $i < $graph_size; $i++) {
  [download]

  and better yet

  for my $i (0..$graph_size-1) {
  [download]

• and and or are designed to be used to join statements. Use && and || inside of expressions.

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