That's because @INC is reset to the contents valid on Apache startup on each run. The use..bla statement is only executed the first time the script is run. On subsequent runs of the same script in the same Apache process (or it's children) the use statements are not executed again.
At least this is my understanding from what I read.

What I did was adding the following line to the end of
/etc/apache2/apache2.conf:

  PerlRequire "/etc/apache2/modperl_startup.pl"
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The file modperl_startup.pl reads:

#!/usr/bin/perl

use lib "/htdocs/perl5lib";

1;
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Hope this helps
RL