Maybe this is what you want ...

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

while(<DATA>) {
    my @tags = ( /(\:TAG\:.*?)[(?=:)]/g );

    for my $tag ( @tags ) {
        print "$tag\n";
    }    
}
__DATA__
:TAG: This is just some text. blahblahblahblahblahblahblah blahblahbla
+h. blahblah?  blah.:TAG: This is just some text. blahblahblahblahblah
+blahblah blahblahblah. blahblah?  blah. :TAG: This is just some text.
+ blahblahblahblahblahblahblah blahblahblah. blahblah?  blah. :TAG: Th
+is is just some more text. blahblahblahblahblahblahblah blahblahblah.
+ blahblah?  blah.
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