Thank you very much!! This solved my problem. I just slurp the text file in a string variable and then apply your regex which I can now fully understand.

#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;


my $file = 'textfile.txt';
open my $fh, '<', $file or die;
$/ = undef;
my $text = <$fh>;
close $fh;

my %res = $text =~ /BEGIN_TAG_(\d+)(.*?)END_TAG/sg;

print Dumper (\%res);
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