Interesting question. I guess you cannot directly use the object as a bind variable since there's no corresponding value in ora_types:

:ora_types
  ORA_VARCHAR2 ORA_STRING ORA_NUMBER ORA_LONG ORA_ROWID ORA_DATE
  ORA_RAW ORA_LONGRAW ORA_CHAR ORA_CHARZ ORA_MLSLABEL ORA_NTY
  ORA_CLOB ORA_BLOB ORA_RSET
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But you could possibly turn the arguments into an XML document and pass that as a CLOB. See this sample from the DBD::Oracle man page:

# Build a large XML document, bind it as a CLOB,
# extract elements through PL/SQL and return as a CLOB

# $dbh is a connected database handle
# output will be large

local $dbh->{LongReadLen} = 1_000_000;

my $in_clob = "<document>\n";
$in_clob .= "  <value>$_</value>\n" for 1 .. 10_000;
$in_clob .= "</document>\n";

my $out_clob;

my $sth = $dbh->prepare(<<PLSQL_END);
-- extract 'value' nodes
DECLARE
    x XMLTYPE := XMLTYPE(:in);
BEGIN
    :out := x.extract('/document/value').getClobVal();
END;

PLSQL_END

# :in param will be converted to a temp lob
# :out parameter will be returned as a string.

$sth->bind_param( ':in', $in_clob, { ora_type => ORA_CLOB } );
$sth->bind_param_inout( ':out', \$out_clob, 0, { ora_type => ORA_CLOB 
+} );
$sth->execute;
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