print "Cannot open file \"$filename\"\n\n";  # Line 16  
 exit;
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You can use die instead of print and exit. Also, die throws exceptions, so they can be catched and handled outside the running code. This can be useful for generating backtraces and other debugging things.

 @all_sentences=(my $sentence);                 # Line 34

Did you mean push? push @all_sentences,$sentence;

 my $diff_letters="code?????";             # Line 38

You can use ... when you don't know what to write, Perl will automatically throw "Unimplemented" exception at this point. Anyway, did you mean for (1..$#all_sentences) {print abs(length $all_sentences[$_-1] - length $all_sentences[$_]), "\n";}? See abs, length, perlsyn for more information.