I believe I've found a bug in Apache::Session, but I'm asking here in case I'm doing something stupid. From the Apache::Session docs:

As you put data into the session hash, Session squirrels it away for later use. When you untie() the session hash, or it passes out of scope, Session checks to see if anything has changed. If so, Session gains an exclusive lock and writes the session to the data store. It then releases any locks it has acquired.

However, it appears that it does not correctly check for differences in an array-of-hashes. I've done a little digging into Apache::Session's tests, and they do not appear to cover more complex data structures.

The series of (long) one-liners below reproduce the bug and show a work-around. I've added comments to make each snippit more clear. Sorry about the long lines.

# Create empty session
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", undef, {FileName=>"/tmp/sessions/test/"}; print D
+umper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d'
        };

# Open that session for preliminary testing
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; print Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d'
        };

# Add simple scalar data to session
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; $s{t} = 1; print Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          't' => 1
        };

# Re-open session to check for added data
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"};print Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          't' => 1
        };

# Start on our AoH
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; $s{m} = [{one => 1}]; print Dumper(\%s); untie 
+%s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          'm' => [
                   {
                     'one' => 1
                   }
                 ],
          't' => 1
        };

# Add another entry to our AoH. Notice that Dumper 
# says it's there before we untie().
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; push @{$s{m}}, {two=>2}; print Dumper(\%s); unt
+ie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          'm' => [
                   {
                     'one' => 1
                   },
                   {
                     'two' => 2
                   }
                 ],
          't' => 1
        };

# Get back the session. Notice that the second entry 
# in the AoH is missing.
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"};print Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          'm' => [
                   {
                     'one' => 1
                   }
                 ],
          't' => 1
        };

# Work-around--create a dummy parameter which forces 
# Apache::Session to realize that the data has changed.
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; push@{$s{m}}, {two => 2}; $s{dummy} = 1; print 
+Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          'dummy' => 1,
          'm' => [
                   {
                     'one' => 1
                   },
                   {
                     'two' => 2
                   }
                 ],
          't' => 1
        };

# Re-open session to check for new data. Notice that 
# the AoH was saved properly due to the dummy param.
#
$ perl -MApache::Session::File -MData::Dumper=Dumper -e 'tie %s, "Apac
+he::Session::File", "41b4260f41720aed6b24e802a41ad57d", {FileName=>"/
+tmp/sessions/test/"}; print Dumper(\%s); untie %s;'
$VAR1 = {
          '_session_id' => '41b4260f41720aed6b24e802a41ad57d',
          'm' => [
                   {
                     'one' => 1
                   },
                   {
                     'two' => 2
                   }
                 ],
          'dummy' => 1,
          't' => 1
        };
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