I'm with cog on this one. I can't make heads or tails as to what you are trying to do here. Note that use of $a and $b should be limited to sort, but that shouldn't cause the "problem" ... if there even is one. I boiled your code down and it seems ... fine?

use strict;
use warnings;
use Data::Dumper;

my $entries = [];
push(@{$entries}, { a => 1, b => undef});
foreach (my $i=0; $i<scalar(@{$entries}); $i++)
{
    print Dumper $entries->[$i];
}
[download]

Although i would rewrite the foreach loop as such:

for my $i (0 .. $#$entries) {
    print Dumper $entries->[$i];
}
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)