#!/usr/local/bin/perl

use strict;
use warnings FATAL => 'all';

sub fill_in_the_blanks {
    my @arr = @_;

    die "No values found" if @arr && 0 == grep $_ ne '_', @arr;

    for (my $left = 0; $left < @arr; $left++) {
        next unless $arr[$left] eq '_';

        my $right = $left;
        $right++ while $right < $#arr && $arr[$right+1] eq '_';

        if ($left == 0) { # NULLs at the left
            $arr[$_] = $arr[$right+1] for $left..$right;
        }
        elsif ($right == $#arr) { # NULLs at the right
            $arr[$_] = $arr[$left-1] for $left..$right;
        }
        elsif ($left == $right) { # single
            $arr[$left] = ($arr[$left-1] + $arr[$left+1]) / 2;
        }
        else {
            $arr[$left ] = $arr[$left -1];
            $arr[$right] = $arr[$right+1];
        }
    }

    return @arr;
}

use Test::Most;

is_deeply [fill_in_the_blanks(qw< >)], [qw< >], "( )";
is_deeply [fill_in_the_blanks(qw<1>)], [qw<1>], "(1)";

throws_ok { fill_in_the_blanks(qw<  _  >) } qr{No values found}, "(  _
+  )";
throws_ok { fill_in_the_blanks(qw< _ _ >) } qr{No values found}, "( _ 
+_ )";
throws_ok { fill_in_the_blanks(qw<_ _ _>) } qr{No values found}, "(_ _
+ _)";

is_deeply [fill_in_the_blanks(qw<1 _ 1>)], [qw<1 1 1>], "(1 _ 1)";
is_deeply [fill_in_the_blanks(qw<1 _ 3>)], [qw<1 2 3>], "(1 _ 3)";

is_deeply [fill_in_the_blanks(qw<1  _ _  3>)], [qw<1 1   3 3>], "(1  _
+ _  3)";
is_deeply [fill_in_the_blanks(qw<1 _ _ _ 3>)], [qw<1 1 2 3 3>], "(1 _ 
+_ _ 3)";

is_deeply [fill_in_the_blanks(qw<1   _>)], [qw<1   1>], "(1   _)";
is_deeply [fill_in_the_blanks(qw<1 _ _>)], [qw<1 1 1>], "(1 _ _)";
is_deeply [fill_in_the_blanks(qw<_   1>)], [qw<1   1>], "(_   1)";
is_deeply [fill_in_the_blanks(qw<_ _ 1>)], [qw<1 1 1>], "(_ _ 1)";

is_deeply [fill_in_the_blanks(qw<4 _ _ _ 5>)], [qw<4 4 4.5 5 5>], "dhu
+ang90";

done_testing;
[download]

The results...

ok 1 - ( )
ok 2 - (1)
ok 3 - (  _  )
ok 4 - ( _ _ )
ok 5 - (_ _ _)
ok 6 - (1 _ 1)
ok 7 - (1 _ 3)
ok 8 - (1  _ _  3)
ok 9 - (1 _ _ _ 3)
ok 10 - (1   _)
ok 11 - (1 _ _)
ok 12 - (_   1)
ok 13 - (_ _ 1)
ok 14 - dhuang90
1..14
[download]

An interesting little problem. It has lots of edge cases and so makes a nice code kata. Thank for posting it.