Marshall:

I was toying with splice and made an amusing variation:

#!/usr/bin/perl -w
use strict;

my $data = '4 NULL NULL NULL 5 1 NULL NULL 2 NULL 6';

my @data = split(/\s+/,$data);

my $iFirstNULL;
my $cur_index=0;

foreach (@data)
{
   if (my $num = /NULL/.../\d/)
   {
     $iFirstNULL = $cur_index if $num == 1;
     splice @data, $iFirstNULL, $cur_index-$iFirstNULL,
                   newVals(@data[$iFirstNULL-1 .. $cur_index])
       if $num =~ /EO/;
   }
   $cur_index++;
}

print "@data\n"; #prints 4 4 4.5 5 5 1 1 2 2 4 6

sub newVals {
    my ($leftVal, $rightVal, $num) = ($_[0], $_[-1], scalar(@_)-2);

    # Left half of the return list is $leftVal, right half is $rightVa
+l
    my @ret = (($leftVal) x int($num/2), ($rightVal) x int($num/2));

    # Insert the average in the center for an odd-sized number of null
+s
    splice @ret, int($num/2), 0, ($leftVal+$rightVal)/2 if $num&1;

    return @ret;
}
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...roboticus

When your only tool is a hammer, all problems look like your thumb.