#!/usr/bin/perl -w
use strict;

while (<DATA>)
{
    process_record() if /^START/;
}

sub process_record
{
   my $line;
   while ($line = <DATA>, $line !~ /^END/)
   {
      print "$line"
   }
   print "\n";  #a printout spacer for next record
}

=prints
These are the first
set of lines
which are to be extracted

These are the second
set of lines
which are to be extracted

=cut

__DATA__

XXXX
YYYY
START
These are the first
set of lines
which are to be extracted
END
XXX
ZZZ
YYY
START
These are the second
set of lines
which are to be extracted
END
aasds
tteret
tertetr