my $UID = getUserIdSomehow();
  $dbh->prepare("SELECT * FROM message 
     WHERE for_user=$UID ORDER BY tstamp");
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Surely you don't prepare the query with a variable in it? You should be using placeholders:

my $UID = getUserIdSomehow();
my $sth = $dbh->prepare("SELECT * FROM message 
     WHERE for_user=? ORDER BY tstamp");
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which then gets executed with $sth->execute($UID)

Tony