#!/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 $num == 1;
     insertValues($iFirstNULL-1,$cur_index,\@data) if $num =~ /E0/;
   }
   $cur_index++;
}

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

sub insertValues
{
   my ($istart, $iend, $aref) = @_;
   my $pos = $istart+1;
   
   my $beginNum = $aref->[$istart];
   my $endNum   = $aref->[$iend];

   while ($aref->[$pos] eq 'NULL')
   {
      if ($pos-$istart < $iend-$pos)   # closer to start
      {
         $aref->[$pos] = $beginNum;
      }
      elsif($pos-$istart > $iend-$pos) # closer to end
      {
         $aref->[$pos] = $endNum;
      }
      else                             # directly in middle
      {
         $aref->[$pos] = ($endNum + $beginNum)/2;
      }
      $pos++;
   }
}