How can you judge from the quoted log entry that parameter h is set? I only can see, that it was set in the referer, but I couldn't see it for the logged request?

Are you using strict and warnings? If not, please do it. If yes, I wonder, that there's no warning?

The following topics are not related to your problem, I think, but maybe give room for "optimization":

If you want to check alternative values of $function, use the if/elsif/else construct. It saves some time (maybe ;o). Otherwise each if will be checked (what for? If $function eq 'load', it cannot be $function eq 'mark').

if ( $function eq 'load' ) {
  # this
}
elsif ( $function eq 'mark' ) {
  # that
}
elsif ( $function eq 'foobar' ) {
  # something completely different
}
else {
  # any other stuff or something unexpected?
}
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Your code:

my $junk;
($junk,my $headsent,my $headword) =  split(/[sw]/,$h);
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could be rewritten like this:

# no $junk anymore and no doubled 'my' !
my ( $headsent, $headword ) = ( split /[sw]/, $h )[1,2];
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Can you reproduce that issue in a little code example which can be tested by the monks to identify the issue?

Updates:

• Is there a difference between the times when it works and when not (other url, special action, whatever?)?