use strict;

while (<DATA>) {
  chomp;
  print padded($_), "\n";
}

sub padded {
  #left-pads enough zeros to a number to ensure that there are at least
  #four digits
  my ($num) = @_;
  my $padded = '0' x (4 - length($num));
  return $padded . $num;
}

__DATA__
23
102
2003
9