Someone else please let me know if this is right, but my understanding was that you w/could do something like the following in a crontab(5) for such a thing:

DBUILDER_DIR=/home/indy/source/dbuilder
* */6 * * * $DBUILDER_DIR/scripts/spamburglar.pl
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To my knowledge, you don't have to put an environment variable definition in a cron entry on the same line, but can put it above entries if it affects more than one, or is more readable that way. Then, in your code, I am thinking you w/could do something like:

unless (defined($ENV{'DBUILDER_DIR'})) {
    $ENV{'DBUILDER_DIR'} = '..';
}
use lib "$ENV{'DBUILDER_DIR'}/lib";
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I haven't used the use lib... pragma you are using before, but I hope this either proves helpful in itself, or results in a more knowledgable monk posting to correct my misunderstanding, enlightening us both. My best in your quest for this knowledge.