Now i can get the data in between <START> and <END>
What do you mean with "get"? (Taking into account merlyn's answer) would something like 
    push @lines, $_ unless $where == 1 or $where =~ /E0/;

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do?
How do i abort the script if there is no <END>?
IIUC you can realize that there's no end "tag" only when you terminate processing your input file. You may just maintain a flag: 
my $foundend;
while (<file>)
 {
   next unless my $where = /START/ .. ($foundend=/END/);
   # ...
 }
print "found\n" if $foundend.

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In reply to Re^2: Between START .. END by blazar
in thread Between START .. END by darrengan

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