in reply to generate random number without using any built in function in perl?
Not to be obtuse, but why can't you use rand? This smacks of an XY Problem. There are a myriad of ways to generate pseudom random series, and the appropriate solution depends very strongly on the intended purpose. 'Best time complexity' is solved by xkcd://221, which is probably not what you mean. Most Linux boxes have /dev/random at their disposal for truly high quality randomness, but this is overkill for nearly every application, and denies the possibility of playback.
Equivalent functionality can be garnered on other systems with TrulyRandom. These are very slow, though still 'Best time complexity'.
~$ perl -e 'open $rnd, "<", "/dev/random"; read $rnd, $value, 4; prin
+t "Value = ", unpack ("H*", $value), "\n"'
Value = 0de13803
Describe your application, and the quality of the guidance we can provide will improve dramatically.
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.