You can certainly "do" an alter session to change the nls_date_format and it sticks with the $dbh you ran it on. Here is an example but using nls_timestamp_format:

perl -e 'use DBI;$h = DBI->connect("dbi:Oracle:XE","xxx","yyy");
$s = $h->prepare("select sys_extract_utc(systimestamp) from dual");
$s->execute;
DBI::dump_results($s);
$h->do("alter session set nls_timestamp_format=\"YYYYMMDD HH24:MI:SS\"
+");
$s = $h->prepare("select sys_extract_utc(systimestamp) from dual");
$s->execute;DBI::dump_results($s);'
[download]

produces:

"27-JUN-07 01.48.13.876853000 PM"
1 rows
"20070627 13:48:13"
1 rows
[download]