Glad you got it working. Couple of suggestions:

• You should always use strict; and use warnings; at the top of your code; Perl provides built-in code validation and it's silly not to use it. One day (many days) it will save your ass.
• You should check to see whether opening a file succeeded.
• You should only declare a variable in the smallest scope possible.
• You might look into Perltidy for help on your code style, which is hard to read and inconsistent. Suggested edit:

#!/bin/perl
use strict;
use warnings;

my @months = ('', qw/jan feb mar apr may jun jul aug sep oct nov dec/)
+; 

print($_, "\t") for @months; 
print "\n"; 

my (%report, $cust);

open(my $DATA, '<', 'sales.txt') or die "Open failed: $!";

while (<$DATA>) {
    chomp; 
    if ( /^[A-Z]/ ) { 
        $cust = $_; 
        @{ $report{$cust} } = map { 0 } @months; 
        next; 
    } 

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

    $report{$cust}[$mon] = $value; 
}

foreach my $cust (keys %report) {
    print $keys,"\t"; 
    print($report{$cust}[$_], "\t") for 0 .. 12; 
} 
print "\n";
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One of the benefits of clear syntax is that it enables you to spot typos, such as the stray

}  
print "\n";
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in your code above.

Hope this helps!