function fIr$Tmp return number is
nRes number;
begin        
  nRes :=  268435456;
  -- nRes := power(2, 28); -- а так все нормально.
  return nRes;
end;

select svc$23532pkg.fIr$Tmp from dual
--------------------------------------
429514

select * from v$version
-------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
CORE    11.2.0.4.0    Production
TNS for Solaris: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production

 return fIr$Tmp.nRes;

alter session set plsql_optimize_level = 0;
alter package svc$23532pkg compile;

select svc$23532pkg.fIr$Tmp from dual
--------------------------------------
429514

select text from dba_source
where owner = 'XXXXXX'
and name = 'SVC$23532PKG'
and line >= 97818
order by line
------------
function fIr$Tmp return number is
nRes number;
 begin        
    nRes :=  268435457;--power(2, 28);
    return nRes;
end;
END SVC$23532PKG;

function fIr$Tmp return number is
nRes number;
 begin        
    nRes :=  268435457;--power(2, 28);
    return nRes;
 end;

function fIr$Tmp2 return number is
cRes CONSTANT number := 268435457;
nRes number;
 begin        
    nRes :=  cRes;
    return nRes;
 end; 

 function fIr$Tmp3 return number is
cRes CONSTANT number := 268435457;
nRes number;
 begin        
    return cRes;
 end;
 
select BP$5024_PO_tmp.fIR$Tmp as ver_0, BP$5024_PO_tmp.fIR$Tmp2 as ver_2, BP$5024_PO_tmp.fIR$Tmp3 as ver_3 from dual
VER_0     VER_2     VER_3
-----------------------------
420057    429514    383429

function fIr$Tmp return number is
nRes number;
 begin        
    nRes :=  268435457+0;--power(2, 28);
    return nRes;
 end;

 function fIr$Tmp3 return number is
cRes CONSTANT number := 268435457;
nRes number;
 begin        
    return cRes;
 end;

select SVC$23532PKG.fIR$Tmp as ver_0, SVC$23532PKG.fIR$Tmp3 as ver_3 from dual
----------------
VER_0    VER_3
429514    268435457

alter session set plsql_code_type = native;

alter session set plsql_code_type = native;
alter package SVC$23532PKG compile;

select SVC$23532PKG.fIR$Tmp as ver_0 from dual
---------------------------
VER_0
429514

select * from USER_OBJECT_SIZE where name = 'SVC$23532PKG'
-------------------------------------------------------------------------
SVC$23532PKG    PACKAGE                55787       160551       91580    0
SVC$23532PKG    PACKAGE BODY    5516957           6848    6959681    0

alter session set plsql_debug = true;
alter package SVC$23532PKG compile debug;

select SVC$23532PKG.fIR$Tmp as ver_0 from dual
VER_0 
------
429514

select * from USER_OBJECT_SIZE where name = 'SVC$23532PKG'

NAME           TYPE    SOURCE_SIZE    PARSED_SIZE    CODE_SIZE    ERROR_SIZE
------------------------------------------------------------------------
SVC$23532PKG    PACKAGE    55787    160551    91580    0
SVC$23532PKG    PACKAGE BODY    5516957    6848    6959449    0

function fXXX return number is
 c1 CONSTANT number := 268435457;
 c2 CONSTANT number := 268435457;
 ...
 cn CONSTANT number := 268435457;
begin 
 return cn;
end;