If you are sure the user ID is an integer: 

my $user_id_h = unpack("H*", pack("Q>",$user_id));
print(dump($user_id_h));

[download]
perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

In reply to Re: Converting integer to bytes by shmem
in thread Converting integer to bytes by Anonymous Monk

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