It really does come down to "it depends on what you are doing." You really have to know your database and your perl very well. In general, I leave most things to the database if possible, and use the SQL to return the smallest, most refined result set back to the script as possible. One thing that I almost always do with perl however, is sorting... SQL cannot touch some of perl's beauty in sorting:

my $unsorted = $dbh->selectall_arrayref($FOOSQL) or 
  die "Fooey, foo failed ($FOOSQL) $DBI::errstr\n";
for (
sort {
  $Q::SortBy == 1 ? 
    ($a->[1] cmp $b->[1] or $a->[4] cmp $b->[4] or $a->[2] cmp $b->[2]
+) :
  $Q::SortBy == 2 ? 
    ($a->[1] cmp $b->[1] or $a->[2] cmp $b->[2]) : 
    ($b->[3] <=> $a->[3] or $a->[2] cmp $b->[2])
}
map {[$_, lc $_->[5], substr($_->[4],3,3), int $_->[1], lc $_->[3]]} @
+$unsorted) {
  ## Do something with $_ here...
}
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