Occasionally, when I have a Heisenbug in a script that resists walk-throughs with 'perl -d', I'll do something like this:

### At the top of the script
my $debug = 1;                  # Set to '0' to disable debugging outp
+ut

sub debug {
    warn "DEBUG ($_[0]): <<<$_[1]>>>" if $debug;
}

[...]

### Some troublesome variable or value
debug('$foobar', $foobar);
[download]

For really stubborn bugs, I open a log file and write it to the log via 'Data::Dumper'. For that class of bug, I haven't yet found one that can beat that particular treatment.

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells