I may be over simplifying things, but this should do:

my ($thingy) = $_ =~ /:::(.*?):::/s;
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There is no global on it so it will get the first one. If for some chance there was more than one that you wanted, I would go with

my (@thingies) = $_ =~ /:::(.*?)(?=:::)/sg;
# ?= allows for it to start at the ::: on the next time through
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I keep seeing people talk about the dot star issue, but the dot star issue is not as much an issue if you have qualifiers before and after that force it to match a specific location.