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

open <ID>, '<', 'textfile.txt';

# a lot of lines later...

my $mystring = "ABC<ID>blahblah</ID>DEF";
if ( $mystring =~ /<ID>(.*?)<\/ID>/) {
  my $ID = $1;
  print "$ID\n";   # OOOPS.... 
}