I can only assume the command being run is the HTMLDoc binary since the HTMLDoc.pm module is a perl interface to HTMLDoc.
The snippet in my original post is only called once (to convert a HTML file to PDF) which would eliminate the possibility of thousands of processes being called.
Additionally this error only occurs on the 'production' server, so the amount of debugging is limited (e.g. almost none).
There seems to be enough swap-space. I've been monitoring it from time-to-time, the below is typical.
swap -l
swapfile dev swaplo blocks free
/dev/swap 0,0 8 8228264 3407016
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