I'm sorry but your post formating is rather illegible. Can you please re-format using <c>...</c> tags around code and <p>...</p> tags for paragraphs of text.

Having said that, the code you have is a little odd but it does work for me. I've tidied it up a bit, moving the lexical (my) variables into the loop where they are used

use strict; 
use warnings; 
use diagnostics; 
use Data::Dumper; 
my %hash; 
my @data = ("foo",1, "bar",2, "foo",3, "baz",4); 
while (my $key = shift @data) { 
    my $value = shift @data; 
    push @{$hash{$key}}, $value; # push the value on the array 
} 
print Dumper \%hash
[download]

and here is the perfectly good output

$VAR1 = {
          'bar' => [
                     2
                   ],
          'baz' => [
                     4
                   ],
          'foo' => [
                     1,
                     3
                   ]
        };
[download]

This output is telling you that the hash has three keys, and each one holds an array reference (thus the [ ]). One of them (foo) holds two values in the referenced array, the others a single value. What are you seeing?

Cheers,
R.