You've failed again to ask your question appropriately.

First the diagnoses:

($sub1, $sub2, $sub3) = split (/\_/, @array,3);
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split doesn't work on an array!!!

while my $elem (@array)
{
  ($sub1,$sub2,$sub3) = split(/_/, $elem, 3);
  # do something here...
}
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Now you might have something in $sub3, but this doesn't make any sense either:

$sub3 = sort { $b <=> $a } $sub3;
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You can't sort a scalar (well, it doesn't do anything interesting)

Seems what you really want to do is sort the elements of @array by the last field (numerically), in which case you'll need something like this:

my @a2;
while my $elem (@array)
{
  my @fields = split(/_/, $elem, 3);
  push @a2, \@fields;
}
my @a3 = sort { $b->[2] <=> $a->[2]} @a2;
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But you should really see How (Not) to Ask a Question.