# 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";