bash-4.1$ printf '%s:%s\n' a b | 
>   while IFS=:; read a b; do 
> printf 'IFS => '
> printf '%s' "$IFS" |
>   od -bc 
> printf '%s => %s\n'  a "$a"  b "$b"  
>   done
IFS => 0000000 072
          :
0000001
a => a
b => b

bash-4.1$ printf '%s:%s\n' a b | 
>   while IFS=: read a b; do 
> printf 'IFS => '
> printf '%s' "$IFS" |
>   od -bc 
> printf '%s => %s\n'  a "$a"  b "$b"  
>   done
IFS => 0000000 040 011 012
             \t  \n
0000003
a => a
b => b

       When a simple command is executed, the shell performs the following expansions, assignments, and redirections,
       from left to right.

       1.     The  words  that  the  parser has marked as variable assignments (those preceding the command name) and
              redirections are saved for later processing.

       2.     The words that are not variable assignments or redirections are expanded.  If any  words  remain  after
              expansion,  the first word is taken to be the name of the command and the remaining words are the argu-
              ments.

       3.     Redirections are performed as described above under REDIRECTION.

       4.     The text after the = in each variable assignment undergoes tilde expansion, parameter  expansion,  com-
              mand substitution, arithmetic expansion, and quote removal before being assigned to the variable.

       If  no  command  name  results, the variable assignments affect the current shell environment.  Otherwise, the
       variables are added to the environment of the executed command and do not affect the  current  shell  environ-
       ment.   If  any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the
       command exits with a non-zero status.

bash-4.1$ a=1; a=2 awk 'BEGIN { printf "%s => %s\n", ARGV[0], ENVIRON["a"] }'; printf '%s => %s\n' "$0" "$a"
awk => 2
bash => 1