You say that the number of tags might increase, so I would probably plan for that from the start and write something generalized like this:

#!/usr/bin/perl -w
use strict;
die "Usage: save-into-arrays.pl input-file.txt\n" unless @ARGV == 1;
my @description;    # where we'll store contents of Description sectio
+n
my @data;           # where we'll store contents of Data section
my %sections = ('Description:' => \@description, 'Data:' => \@data);
open(INPUT_FILE, "<$ARGV[0]") or die "Unable to open $ARGV[0]: $!\n";
my $array_ref;      # keeps reference to array currently in use
while (my $line = <INPUT_FILE>) {
  chomp $line;
  if (defined($sections{$line})) {          # is this is a section hea
+d?
    $array_ref = $sections{$line};          # yes, so point to the arr
+ay for this section
  } else {
    push @$array_ref, $line if $array_ref;  # no, save this line in th
+e active array
  }
}
close(INPUT_FILE);
[download]

Basically what this does is create a hash that pairs each possible section head with an array where you'll store the data from that section. When a section head is encountered, $array_ref gets updated to point to the right array for subsequent lines. This way when you have new section tags, you can just create a new array and a new entry for it in the hash -- no need to alter the underlying logic with more if statements, etc.

Cheers...