For some reason though, I can't get it to work. Here's the code I'm using.

#!/usr/bin/perl

use warnings;
use strict;

my $input_data  = "real.txt";
my $output_data = "output.txt";

open(FILE, $input_data) or die "Cannot open file $input_data because: 
+$!";
my @text = <FILE>;
close(FILE);

foreach my $line (@text)
{

  if ( $line =~ /([A-Z]+) *- *([^-]+) *- *([^"]+)"([^"]+)".+?([0-9]+)$
+/sm )
  {
    print "CATEGORY $1\n\n";
    print "KEYWORDS $2\n\n";
    print "Summar text: $3\n\n";
    print "Reference: $4\n\n";
    print "Id: $5\n\n";
  }
  else
  {
    print "none";
  }
}
[download]

ABETALIPOPROTEINEMIA          

ABETALIPOPROTEINEMIA - Vitamin A, Vitamin E - In a study of 10
subjects who were 3-25 years of age (mean age 14.6 years) who were
diagnosed with abetalipoproteinemia during their first year of life
and from then on received 100 mg of vitamin E/kg and 10,000-15,000
IU/day of vitamin A compared with 10 age-matched control subjects,
levels of plasma carbonyls did not differ significantly between
patients and controls.  The lag phase of plasma oxidizability was
28.03 minutes in the treated subjects compared with 24.0 minutes in
the healthy subjects.  Cyclic voltammetry showed a peak potential
of 330 mV in all the samples studied, which suggests that the same
antioxidants were present in the plasma of the patients and the
control subjects.  The anodic current of the samples, which
measures the concentrations of hydrophilic low-molecular-weight
antioxidants, was 5.227 versus 5.38 uA in the patients and control
subjects, respectively.  These data suggest that enhanced oxidative
stress is not apparent in the plasma of abetalipoproteinemia
patients receiving long-term supplementation with vitamin A and E. 
It is noted that the neurologic and ophthalmologic symptoms of
abetalipoproteinemia are believed to be, in part, caused by alpha-
tocopherol deficiency.  "Oxidative Stress in Abetalipoproteinemia
Patients Receiving Long-Term Vitamin E and Vitamin A
Supplementation," Granot E, Kohen R, Am J Clin Nutr, 2004;79:226-
230.   (Address:  Esther Granot, E-mail: etgranot@md.huji.ac.il) 
41401

ABORTION

ABORTION - Spontaneous, Caffeine - In a review of 15 epidemiologic
studies, all of which suffered from important methodologic
limitations, most reported a positive association between maternal
caffeine intake and the risk of spontaneous abortion.  However, the
authors conclude that the evidence must be considered equivocal,
since these biases would tend to overestimate any association. 
"Maternal Caffeine Consumption and Spontaneous Abortion: A Review
of the Epidemiologic Evidence," Signorello LB, McLaughlin JK,
Epidemiology, March 2004;15(2):229-239.  (Address:  Joseph K.
McLaughlin, E-mail: jkm@iei.ws) 41684
[download]

well. you just canīt apply a regex that is suppused to match multiple lines repeatedly to single lines. that wonīt ever work.

this will do the job:

#!/usr/bin/perl

use warnings;
use strict;

my $infile  = "input.txt";
my $outfile = "output.txt";
my $text;

{
    local $/=undef; #set line separator locally to undef to read in th
+e file as a whole
    open(FILE, $infile) or die "died opening $infile for input: $!\n";
    $text = <FILE>;
    close(FILE);
}

open OUT, ">$outfile" or die "died opening $outfile for output: $!\n";

#repeatedly match searched text, while deleting found occurences 
#to avoid endless loop

while ( $text =~ s/([A-Z]+) *- *([^-]+) *- *([^"]+)"([^"]+)".+?([0-9]+
+)$//sm )
{
    print OUT "CATEGORY $1\n\n",
              "KEYWORDS $2\n\n",
              "Summar text: $3\n\n",
              "Reference: $4\n\n",
              "Id: $5\n\n",
}

close OUT;
[download]

i suggest reading perlre

Update:

this one is better because it uses \G to avoid unneccessary backtracking

#!/usr/bin/perl

use warnings;
use strict;

my $infile  = "input.txt";
my $outfile = "output.txt";
my $text;

{
    local $/=undef; #set line separator locally to undef to read in th
+e file as a whole
    open(FILE, $infile) or die "died opening $infile for input: $!\n";
    $text = <FILE>;
    close(FILE);
}

open OUT, ">$outfile" or die "died opening $outfile for output: $!\n";

#repeatedly match searched text, using \G
#to avoid endless loop

while ( $text =~ m/\G.*?([A-Z]+) *- *([^-]+) *- *([^"]+)"([^"]+)".+?([
+0-9]+)$/gsm )
{
    print OUT "CATEGORY $1\n\n",
              "KEYWORDS $2\n\n",
              "Summar text: $3\n\n",
              "Reference: $4\n\n",
              "Id: $5\n\n",
}

close OUT;
[download]