Your do is passed along to print as a list which takes the list, inserts a space between the items, and prints it out. Also since you are assiging a value to $i it is passed along to print to be printed also.

Additionally, you are loading the entire file into your programs memory space and then tossing all but the first 30 lines.

#starting from the open
print header;
print start_html('A Simple Example');
my $i=0;
while(<INPUT>)
{
  $i++;
  last if $i>30;
  print;
}
close(INPUT);
print end_html;
[download]

I tried to make this example similar in spirit to the original. This method only reads the first 30 lines and only uses memory for one at a time. The file you are printing is not html though so it will still not look right at all in a browser. You may want:

print header('text/plain');
#the while here
#no start or end html prints
[download]

This will print the contents as a text file in which the line breaks and spacing from the file will be preserved.