The DBI->trace output showed me that mysql_auto_reconnect was on when the script was run by Apache.

Note that this is true even if you don't have Apache::DBI (which completely overloads disconnect), and even if you don't have mod_perl. DBD::mysql checks for MOD_PERL or GATEWAY_INTERFACE in the env, and if it sees them, turns on mysql_auto_reconnect on the handle when you connect. (perldoc DBD::mysql for details.)

So in my case I needed to explicitly set  $dbh->{mysql_auto_reconnect} = 0 and then call $dbh->disconnect. Otherwise, when I "verify that it's disconnected" with the ping() call, I actually reconnect first. (The ping() was really just something I was doing to make sure that Apache::DBI hadn't snuck itself back in--a debugging device.)

Thanks again for help everyone.

Oh, also, in comparing two DBI->trace outputs I wrote a hack to help diff ignore the differences in memory addresses. Here it is:

#!perl -w

#use like this, if file1 and file2 are two traces of similar db operat
+ions:
# perl ox2read <file1 >file1scrub
# perl ox2read <file2 >file2scrub
# diff -c file1scrub file2scrub >mydiff

use strict;
my @seen_addy = ();
while (my $line = <>) {
    foreach my $sr ( @seen_addy ) {
        my $search = $sr->[0];
        my $replace = $sr->[1];
        $line =~ s/$search/$replace/g;
    }   
    while ($line =~ /(0x\w+)/) {
        my $search = $1;
        my $last_replace = @seen_addy ? $seen_addy[$#seen_addy]->[1] :
+ 'addy001';
        my $replace = ++$last_replace;
        push @seen_addy, [$search, $replace];
        $line =~ s/$search/$replace/g;
    }
    print $line;
}
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