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

my $text = <<EOT;
Any text is in this document.
But I'm only interested in the text between a beginning and an end tag.
BEGIN_TAG_110
In this text I'm interested in.
It can consist of several lines. Until the end tag.
END_TAG and any other things I'm not interested in until the next beginning tag starts.
Any other text.
BEGIN_TAG_237
I need this text.
END_TAG
More text ...
EOT

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

print Dumper (\%res);