my @codes = (
  '12345678',
  '1234567890123',
  '1234567890123456',
);

my $len = 13;

for my $var (@codes) {
  my $fixed = substr $var . '0' x $len, 0, $len;
  print "$fixed\n";
}

__END__
1234567800000
1234567890123
1234567890123