While adding the length check is valid for a pure coding solution, the most practical reason where leading zero's are necessary is for values that are required to be a fixed length such as SSN's.

If a value initially  exceeded that fixed length, it most likely would have been trapped out as an invalid entry before it reached the length adjustment code.

This answer  by Pavel is the simplest way. :)

Try this:

ROU>s val="sd123X"             ; value to be justified

ROU>s len=9                             ; length of the total string

ROU>k z                                      ; make sure z is undefined

ROU>s $P(z,"0",len)=val        ; set z to value with leading zeros

ROU>s val=$E(z,*-(len-1),*)   ; trim value to required length

ROU>w !,val

000sd123X