use strict;
use warnings;

my $dat0 = 'a.txt';

open (DAT, "$dat0") or die "Could not open file `$dat0'.\n";
my @all=<DAT>;
close (DAT);

my @words = ();
my $sentences = {}; # 'word' => [ sentences that start with `word' ]
foreach my $sentence (@all) {
    chomp ($sentence);
    push (@words, [ split (/[ \t]+/, $sentence) ]);
    my $firstWord = $words[-1]->[0];
    $sentences->{$firstWord} = [] if not exists $sentences->{$firstWor
+d};
    push (@{$sentences->{$firstWord}}, $#words);
}

my @temp = ('', '');
for (my $i = 0; $i <= $#words; $i++) {
    push (@temp, $all[$i]);
    my @referencedSentences = ();
    foreach my $j (@{$words[$i]}) {
        if (($j ne "$j") || ($j ne "v")) { # I don't get this so I lea
+ve it intact
            if (exists $sentences->{$j}) {
                push (@referencedSentences, $sentences->{$j});
            }
        }
    }
    push (@temp, \@referencedSentences);
}

print "Done.\n";

# ...
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• Initialize an empty list with my @list = ();, and not @list = '', which initializes the first element to an empty string.
• Always use my, always stick to warnings and strict.
• Avoid recalculating the same values more than a couple of times by caching them (like split in your code).
• Use descriptive names and comments, especially when asking for assistance :)

$dat0 = 'a.txt';
open( DAT, "$dat0" ) || die ("Could not open file!");
@all  = <DAT>;
# I'm not enven sure what perl will do when you open the same file han
+dle
# twice without closing, I would assume it would close the first for y
+ou,
# but it's just bad.
open( DAT, ">$dat0" ) || die;
# You never write back to @all so you are doing nothing.
print DAT @all;
close(DAT);
[download]

use strict;
use warnings;
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If you do that it would be easier to help.

He's not using $1 (one), he's using $l (lower case L). Granted, it looks very confusing. Confusing enough to discourage its use.

ps : this script is supposed to get all the lines from a file and refer each word to the sentence that starts with that word (and, there aren't 2 sentences that start identically).

Does the input data file really contain exactly one "sentence" per line? Are you certain that the "words" in each sentence are always separated by exactly a single space character? Are the words in "mixed case", and do they include punctuation marks? (And does this have an effect on what you are trying to do?) Why should it matter if a sentence contains a "word" that consists of the single letter "v"?

Let's suppose a particular word (e.g. "bar") occurs at the beginning of one sentence (e.g. sentence #23), and also occurs in the middle or at the end of 4 other sentences (e.g. #5, #12, #47, #69). What do you want to accomplish with regard to this word? Locate just the one sentence that begins with "bar"? Locate just the other four sentences that contain "bar"? Locate all five sentences (and identify the one that begins with "bar")? What do you want to do with words that only occur in the middle or at the end of sentences but never at the beginning of any sentence? Ignore them?

How you answer those questions will determine how you should read through the sentences and words, what sort of data structure you should create from the input data, and how you would use that data structure after you've built it.

As for the code you posted at the start of this thread, the reason it takes so long for more sentences is the nesting of your "for" loops:

  foreach sentence in the file {
    ...
    foreach word in the sentence {
      ...
      foreach sentence in the file {
        ... # given n sentences with and avereage of m words each,
            # this block has to execute n*m*n times
      }
    }
  }
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