You're still missing these two fundamental lines:

use strict;
use warnings;
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at the beginning of your program. Get into the habit to always include them, because they will help you spotting various errors and pitfalls.

Regarding loading files, note that you had to cut-and-paste the code while you could have factored it all out into a function. Moreover, IMHO you're using constructs that I'd avoid. If you're able to install modules in your system, you could install File::Slurp and do something along these lines:

use File::Slurp qw( read_file );
my $projectA = read_file('projecta.txt');
my $projectB = read_file('projectb.txt');
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If you want to roll your own, implement the read_file() sub yourself:

sub read_file {
   my $filename = shift;

   # Force whole file into one scalar unless we want
   # each "line" on its own
   local $/ unless wantarray;

   open my $fh, '<', $filename # USE 3-args version of open!
      or die "open('$filename'): $!"; # verify your open!

   return <$fh>; # enjoy auto-close of $fh :)
}
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Regarding the split into sentences, note that there are other sentence terminators (question and exclamation marks, to name a few). Last, but not least, please re-read my previous post about your algorithm.