# Save input from STDIN into $file
my $file = <STDIN>;

# If $file matches one letter or number, then underscore
# and then .dat, then do the block.  By the way, you are
# missing the closing bracket for the block. :)
if ($file=~/[a-zA-Z0-9_]\.dat/){

# Open file or die with some error.
open(DATOS, "$file") || die "No se puede abrir el archivo, o no existe
+ en el directorio";

# Read file line by line
while (<DATOS>){

# Initialize array @lines
my @lines;

# Initialize array @line and make it equal to an
# empty array @lines, if the line from file starts with 
# the digit.
my @line = @lines if m/^\d/;

# Print empty array @lines
print "@lines";
[download]

# Initialize array @lines
my @lines;

while (<DATOS>) {

 # Add line from file to array @lines if line starts with 
 # digit
 push @lines, $_ if m/^\d/;

}

# Print array @lines after all processing has been done.
print @lines;
[download]

if ($file=~/[a-zA-Z0-9_]\.dat/){
[download]

The [a-zA-Z0-9_] can be shortened to \w. Guessing from your error message and some of your variable names, English isn't your first language, and I bet your system will allow characters in filenames that aren't on an English keyboard, in which case the regex you have above won't match all valid filenames (in fact, it won't even on an English-native system, since almost everyone these days allows spaces in filenames). \w will probably respect your locale, though I haven't checked it.

Actually, you can get around all of this by using -e instead of that regex, which will check for the existance of a file. So that if statement becomes:

if( -e $file ) {
[download]

my $file = <STDIN>;
chomp($file);

if ($file =~ /\w+\.dat/) {
   open(DATOS, "$file") or die "error message";
   while (<DATOS>) {
      push(@lines, $_) if /^\d/;
   }
   close DATOS;
   foreach my $line (@lines) {
       my @fields = split(/\s+/, $line);
       print "@fields";   # or whatever else you want to do
   }
}
[download]

Your @lines and @line arrays will always be empty with that code. I don't understand where you think you are getting a hexadecimal value from.

It may be easier if you ask your question in your native language. It is likely that someone here will be able to answer you or at least translate your question.

-sauoq
"My two cents aren't worth a dime.";