Maybe you could avoid the regex approach and use split and join?

#  untested
my @parts = split /-/, $input;
my $first = shift @parts;
my $last  = pop @parts;

my $reassembled;

# then
$reassembled = $first . '-' . join ('_', @parts) . '-' . $last;
#  or something like this 
$reassembled = join '_', @parts;
$reassembled = join '-', ($first, $reassembled, $last);
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The above assumes no CSV type quoting issues with embedded separators, for which a proper CSV parser like Text::CSV is needed.

I see a typo in my description ...

If you have not already done so (I don't see any Update), you might consider updating the errant node to clarify the mistake. Please see How do I change/delete my post?