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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