Be sure to set it bbefore/b DBD::Oracle connects, like mje suggests

Be sure to have your data correctly encoded before passing it to the DBI. See what happens with the different data:

$ perl -C2 -MDP -wle'$_="\xc3\x84";print"$_\t",DPeek$_'
à      PV("\303\204"\0)
$ perl -C2 -MDP -wle'$_="\xc3\x84";utf8::decode$_;print"$_\t",DPeek$_'
Ä       PV("\303\204"\0) [UTF8 "\x{c4}"]
$ perl -C2 -MDP -wle'$_="\x{c4}";print"$_\t",DPeek$_'
Ä       PV("\304"\0)
$ perl -C2 -MDP -wle'$_="\x{c4}";utf8::decode$_;print"$_\t",DPeek$_'
Ä       PV("\304"\0)
$ perl -C2 -MDP -wle'$_="\x{c4}";utf8::encode$_;print"$_\t",DPeek$_'
à      PV("\303\204"\0)
$ perl -C2 -MDP -wle'$_="\x{c4}";utf8::encode$_;utf8::decode$_;print"$
+_\t",DPeek$_'
Ä       PV("\303\204"\0) [UTF8 "\x{c4}"]
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