Is this what you're looking for?

use strict;
my $max = 0;
foreach (<DATA>) {
   s/(\r|\032)$//;
   $max = length($_) if length($_) > $max;
}
print " " x $max;

__DATA__
This is my file
it doesn't have
a blank line at the end
of it based on
the maxline count of the file
that it read.
[download]

Cheers,
Shendal