The DBD::Oracle documentation specifies that one can (not saying should) use bind_param_inout for stored functions.

Snippet from DBD::Oracle docs (comes installed with DBD::Oracle, search for DBD/Oracle.html in your Perl source tree)

# Example 4
#
# What about the return value of a PLSQL function?
# Well treat it the same as you would a call to a function 
# from SQL*Plus. We add a placeholder for the return value 
# and bind it with a call to bind_param_inout so
# we can access it's value after execute.

  my $whoami = "";
  $csr = $db->prepare(q{
        BEGIN
            :whoami := PLSQL_EXAMPLE.FUNC_NP;
        END;
  });
  $csr->bind_param_inout(":whoami", \$whoami, 20);
  $csr->execute;
  print "Your database user name is $whoami\n";
  $db->disconnect;
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Jorg

"Do or do not, there is no try" -- Yoda