my @array = <DATA>;
chomp;
for my $array (@array){
    $array =~ s/\n$// if ($array !~ /;[^;]*;/) && ($array =~ /^\w/);
    $array =~ s/^\s*([^\s])/$1/;
}
print @array;

__DATA__
The Road Ahead1;Completely Revised and 
               Up-to-Date; 002564418
road ahead2; America?~@~Ya creeping revolution;00345678
The road ahead3;[Address made before the Regional 
         Foreign Policy Conference; 004561963
[download]

The Road Ahead1;Completely Revised and Up-to-Date; 002564418
road ahead2; America’a creeping revolution;00345678
The road ahead3;[Address made before the Regional Foreign Policy Confe
+rence; 004561963
[download]

Thank you very much!

use warnings;
use strict;

my @records;

$/ = ';';

while (<DATA>) {
    chomp;
    s/\s*(?<!^)\n\s*/ /mg;
    push @records, $_;
}

print join "\n", @records;

__DATA__
The Road Ahead1;Completely Revised and
               Up-to-Date; 002564418
road ahead2; America’a creeping revolution;00345678
The road ahead3;[Address made before the Regional
         Foreign Policy Conference; 004561963
[download]

Prints:

The Road Ahead1
Completely Revised and Up-to-Date
 002564418 road ahead2
 America’a creeping revolution
00345678 The road ahead3
[Address made before the Regional Foreign Policy Conference
 004561963
[download]

which may provide a useful starting point.

I recommend the idiom $_ .= <>. For example, in this case this would be something like:

use warnings; use strict;
while (<DATA>) {
        while (!/(?:[^;]*+;){2}/) {
                $_ .= <DATA>;
        }
        print "read one record: \n(($_))\n";
}
__DATA__
The Road Ahead1;Completely Revised and
               Up-to-Date; 002564418
road ahead2; Americaa creeping revolution;00345678
The road ahead3;[Address made before the Regional
         Foreign Policy Conference; 004561963
[download]

You may need to modify the regular expression to whatever it is that you recognize a full record.

If the continuation lines are marked by the whitespace at the beginning of the next line (like in mail headers) not somehoe in the incomplete line (like in smtp commands), then you can't use this simple idiom. In this case, I recommend a one-line buffer, e.g.

use warnings; use strict;
my($buf, $bufq);
sub peekline {
        $bufq or $buf = <DATA>;
        $bufq = 1;
        $buf;
}
sub getline {
        $bufq or $buf = <DATA>;
        $bufq = ();
        $buf;
}
while (defined($_ = getline)) {
        while (defined(peekline) and peekline =~ /^\s/) {
                $_ .= getline;
        }
        print "read one record: \n(($_))\n";
}
[download]

Yes, Ambrus. I think the best way is to recognize a full record and make whatever in between as one record. The input file is in fact shown as below, The first element of one record should be a file name:

file1.xml;Description A, B
    C, D, E...;SystemNumber
file2.txt;Description X,Y,Z;SystemNumber
file3.xml;Description M,N,
          O,P,Q;SystemsNumber
[download]

I think it might be easy if I first delete all the white spaces in each line; then read all lines into an array as the elements; then split the array by the ".xml" or ".txt" files and put them as the first element of a child array. Is that the right track?