__DATA__
CUSTA
1/2015 100
12/2015 1,000
CUSTB
2/2015 100
3/2015 100

##</code><code>##

$VAR1 = {
          'CUSTA' => [
                       undef,
                       '100',
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       '1,000'
                     ],
          'CUSTB' => [
                       undef,
                       undef,
                       '100',
                       '100'
                     ]
        };


##</code><code>##

    # note:
    #
    # @{ ... } turns the hash entry for this customer into an array.
    #
    # map creates a new list consisting of one element, undef, for each
    # element from the list it is given, i.e. @months

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Sortkeys = 1;

my @months = qw/dummy jan feb mar apr may jun jul aug sep oct nov dec/;

my %report;
my $cust;

while (<DATA>) {
  chomp;
  if ( /^[A-Z]/ ) {
    $cust = $_;
    @{ $report{ $cust } } = map { undef } @months;
    next;
  }

  my ($date, $value) = split ' ', $_;
  my ($mon, $year)   = split '/', $date;

  $report{$cust}[$mon] = $value;
}
print Dumper(\%report);

__DATA__
CUSTA
1/2015 100
12/2015 1,000
CUSTB
2/2015 100
3/2015 100

##</code><code>##

$VAR1 = {
          'CUSTA' => [
                       undef,
                       '100',
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       '1,000'
                     ],
          'CUSTB' => [
                       undef,
                       undef,
                       '100',
                       '100',
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef,
                       undef
                     ]
        };