if($u=~/^\d{4}$/){$u=~s/\d\d\d\d/00$u/;}
if($u=~/^\d{5}$/){$u=~s/\d\d\d\d\d/0$u/;}
# but better written as:
#  $u = "00$u" if $u =~ /^\d{4}$/;
#  $u = "00$u" if length($u) == 4;  # if you know it's digits