This does what you want in a one liner. The loop is a little easier to understand if you are unfamiliar with map but they are essentially identical.

I have also shown you how to make your data access a lot less rigid. This code does not care about the structure of your 2D array which has been made irregular.

Not sure what you mean by access a 2D array as 1D. Do you want to flatten it into a list? I have coded that for you. Otherwise it is a 2D structure.

tachyon

#!/usr/bin/perl -w

use strict; 

# here we have an irregular sized 2D array
my @array = ( [1],[2,2],[3,3,3],[4,4,4,4] );
my $all ='42';

# this sets all elements to $all
for (@array) {
  @$_ = ($all) x @$_;
}

# this also sets all elements to $all
# map is just the loop above in short form 
map{ @$_ = ($all) x @$_ }@array;

# this prints the entire structure
for (@array) {
  for (@$_) {
      print "-",$_;
  }
  print"\n";
}
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Explanation. $_ is magical. When we loop over @array each element is aliased to $_ in turn if we modify $_ we modify the actual element @$_ derefs our array elements in list context and allows us to modify them @$_ in scalar context gives the number of elements thus the polymorphic behaviour in this example

If you want to flatten this 2D array into a 1D array this code does that

map{ push @list, @$_ }@array;
print "@list";
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