What version of perl are you using? 5.8.8 (msys) / 5.8.9 (AS 826) both exhibit this behavior. 5.18.2 Strawberry does not.

#!perl
use strict;
use warnings;

my %hash = (
        "A" => 1,
        "B" => 1,
        "C" => 1,
        "D" => 1,
        "E" => 1,
        "B" => 1,
        "F" => 1,
        "G" => 1,
        "H" => 1,
        "I" => 1,
        "J" => 1,
);

for my $key (sort(keys(%hash))) {
        print "$key\n";
}

print "_"x10,"\n";

my %hash2 = makesubhash();

for my $key (sort(keys(%hash2))) {
        print "$key\n";
}

sub makesubhash {
        my %subhash = (
                "A" => 1,
                "B" => 1,
                "C" => 1,
                "D" => 1,
                "E" => 1,
                "B" => 1,
                "F" => 1,
                "G" => 1,
                "H" => 1,
                "I" => 1,
                "J" => 1,
        );
}
[download]

Perl 5.8.8/5.8.9

A
B
C
D
E
F
G
H
I
J
__________
B
C
D
E
F
G
H
I
[download]

Perl 5.18.2

A
B
C
D
E
F
G
H
I
J
__________
A
B
C
D
E
F
G
H
I
J
[download]