#!/usr/bin/perl

#use warnings;
use strict;

my @months = ( '','jan','feb','mar','apr','may','jun','jul','aug','sep
+','oct','nov','dec' );
my @CUSTOMERS; # retain order of appearance in Crystal reports file.
my %DATA; # hash, as suggested by fellow monks

my $customer = ""; # holds current customer
my ($month, $day, $value);
while(<DATA>){
    chomp;
    next unless $_; # skip empty lines
    # sorry, others did a better job with a simple split.
    if( m{^\s*(\d+)/(\d+)\s+([\d,\.]+)} ){
        # matches a line with date and value
        ($month,$day,$value) = ($1, $2, $3);
        # 0+$month transforms string 02 into value 2
        $DATA{$customer}{0+$month} = $value;
    }else{
        $customer = $_; # next customer
        push @CUSTOMERS, $customer;
        # map{ $DATA{$customer}{$_}=0 } (1..12) # uncomment for zero's
    }
}

print join(";", @months). "\n";
for my $customer (@CUSTOMERS){
    print $customer 
        .";"
        . join( ";", 
            map{ $DATA{$customer}{$_} } (1..12)
        )
        . "\n";
}



__DATA__
CUSTA
1/2015 100
10/2015 1,000
12/2015 1,000
CUSTB
02/2015 200
10/2015 1,000
12/2015 100
[download]

output:

;jan;feb;mar;apr;may;jun;jul;aug;sep;oct;nov;dec
CUSTA;100;;;;;;;;;1,000;;1,000
CUSTB;;200;;;;;;;;1,000;;100
[download]