How about this?

#!/usr/bin/perl

use strict;
use warnings;

my @data = ("NULL", "NULL", 1, 3, 70, "NULL", "NULL", "NULL", "NULL", 
+"NULL", 50, 1, "NULL", 4, "NULL", "NULL", 5, "NULL", "NULL", "NULL");

my @data_indices = grep { $data[$_] ne "NULL"; } (0 .. $#data);

if (@data_indices) {
    # Phase 1, fill the edges
    if ($data_indices[0] > 0) {
        # Left edge
        map { $data[$_] = $data[$data_indices[0]]; } (0 .. $data_indic
+es[0] - 1);
    }
    if ($data_indices[$#data_indices] < $#data) {
        # Right edge
        map { $data[$_] = $data[$data_indices[$#data_indices]]; } ($da
+ta_indices[$#data_indices] + 1 .. $#data);
    }

    # Phase 2, fill all gaps between values
    for my $index (1 .. $#data_indices) {
        my $difference = $data_indices[$index] - $data_indices[$index 
+- 1];
        if ($difference == 1) { next; }
        # fill the first half with the leftmost data-element
        map { $data[$_] = $data[$data_indices[$index - 1]]; } ($data_i
+ndices[$index - 1] + 1 .. $data_indices[$index] - $difference / 2);  
+  #.. truncates, so /2 will work.
        # ... and the last half with the rightmost data-element
        map { $data[$_] = $data[$data_indices[$index]]; } ($data_indic
+es[$index - 1] + 1 + $difference / 2 .. $data_indices[$index] - 1);  
+  #.. truncates, so /2 will work.
        if (($difference % 2) == 0) {
            # This gap does have an equidistant data-element (which wa
+s left-filled, so we'll need to overwrite)
            my $middle_index = $data_indices[$index] - $difference / 2
+;
            $data[$middle_index] = ($data[$middle_index - 1] + $data[$
+middle_index + 1]) / 2;
        }
    } ## end for my $index (1 .. $#data_indices)
} ## end if (@data_indices)

print (join (',', @data) . "\n");
[download]

This first indexes all non-NULL values, and then fixes the NULL-values step by step.
I'm using map a lot, but you could implement that using foreach-loops if you find map tricky to parse.

Edit: added if (@data_indices) to skip all work if @data contains only "NULL"-values or is empty.
Edit2: That was silly. Replaced @data-to-hash-to-ordered-array with a single grep.

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