Well, searching "range from list" reveals the thread: [Solved] Converting a list of numbers to use a range operator, which suggests using Set::IntSpan's run_list method:

my @array = (2,3,4,10,11,12);

my $set = new Set::IntSpan join(',', @array);
my $list = $set->run_list;

# $list is now "2-4,10-12"
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