use parens in the pattern, as per perldoc -f split. :

my $foo = <<HERE
:TAG:This is just some text. blahblahblahblahblahblahblah blahblahblah
+. 
blahblah?  blah.:TAG:This is just some text. blahblahblahblahblahblahb
+lah blahblahblah. 
blahblah?  blah. :TAG:This is just some text. blahblahblahblahblahblah
+blah blahblahblah. 
blahblah?  blah. :TAG:This is just some more text. 
blahblahblahblahblahblahblah blahblahblah. blahblah?  blah. 
HERE
;
@foo = split /(:TAG:)/,$foo;
print join "\n",@foo
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