I'm not exactly sure what you want because I can interpret your text several ways. Here's a couple of snippets of code though. The first assumes that you really want something like this:

====header info====
header
header
====header info====
data
data
====header info====
header
header
====header info====
data
....
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... and so on. While the second assumes that you just want the stuff that's in between the ====header info==== lines while discarding the header lines themselves. The second one is what I believe most people interpret your text to mean, but I thought I'd mention the first one just in case (also, it's a rare chance that I get to use the ... (yes, that's 3 dots!) flip-flop operator ;-)

#!/usr/bin/perl

# snippet number 1

while (<DATA>) {
   if (/^====header/.../^====header/) {  print "header: $_"; next; }
   print "data: $_"; next;
}

__DATA__
====header info====
10 to 50 line of text and
numbers with irregular
formatting
====header info====
10 to 50 lines of text and...
More text more text
====header info====
10 to 50 line of text and
numbers with irregular
formatting
====header info====
10 to 50 lines of text and...
More text more text
[download]

#!/usr/bin/perl

# snippet number 2

my (@records,@tmp);
while (<DATA>) {
   chomp;
   if (/^====header/) {
      next unless @tmp;
      push @records, [ @tmp ];
      @tmp = ();
      next;
   }
   push @tmp, $_;
}
push @records, [ @tmp ] if @tmp;
print "@$_\n" for @records;

__DATA__
====header info====
10 to 50 line of text and
numbers with irregular
formatting
====header info====
10 to 50 lines of text and...
More text more text
====header info====
10 to 50 line of text and
numbers with irregular
formatting
====header info====
10 to 50 lines of text and...
More text more text
[download]