I'm suspicious of the "# Some more processing here which is not relevant" section..

Ok, well for the record - here are the lines that I removed:

my $icon = "$themepath/$status.gif";
my $testdetails = "/bb/html/$host.$test.html";
print qq(\t<th><a href="$testdetails">),
   qq(<img src="$icon" border="0" alt="$status" $iconsize title="$test
+: $status since $duration ago">),
    qq(</a></th>\n);
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Basically, I was getting a warning anytime I tried to use $status, obviously because it was undefined.

Anyway, since posting earlier I've been out for lunch and cleared my head a bit and just taken another look at it. And I've found that if I change the conditional in the if statement to:

if (defined $tref->{$host}{$test}{status}) {
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...the warnings disappear.

Although this has solved my immediate problem, I'm still puzzled as to why..

defined $tref->{$host}{$test}
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..returns true in this case, yet

defined $tref->{$host}{$test}{status}
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..returns false - when I know for a fact (from the dumper output) that $tref->{$host}{$test} does not exist.

Thanks for your feedback :)