use 5.010;
use strict;
use warnings;

$/ = undef;
my $data = <DATA>;

#method 1: build regex on the fly (read tags from files)
say '---------- method 1 ----------';
my @tags = qw(
                 TAG1
                 TAG2
                 TAG3
                 TAG4
            );

my $tags_re = join "|", @tags;
$tags_re = qr{ $tags_re };
say $tags_re;

my @stuff = split /($tags_re)=\s*/, $data;
say "#$_#"      for @stuff;

# method 2: static regex
say '---------- method 2 ----------';
my @stuff2 = split /(TAG1|TAG2|TAG2|TAG4)=\s*/, $data;
say "#$_#"      for @stuff2;

__DATA__
TAG1= data
TAG2= more data
TAG3= even more data that sometimes has = and
runs on to more
than one line
TAG4= still more