And for the file:

2
2
2
3
3
3
4
4
5

How would that be handled?

this    or  that?
2             2
3             3
4             2
5             3
2             4
3             2
4             3
2             4
3             5

Fundamentaly whay you need is this:

# Create an array of numbers with values...
my %numbers = ();
for (<FILE>){
   chomp;
   $numbers{$_}++; # tally the occurences of numbers..
   # Could add a preemptive 'last' if numbers not contiguous
}

while( %numbers ){
  # satisfies "this" criteria, insert a reverse for "that"
  for ( sort keys %numbers ){
    print "$_\n";
    delete $numbers{$_} unless --$numbers{$_};
    # Add a routine here to remove all higher values
    # and warn if numbers are non-contiguous.
  }
}
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If this is homework, please note we're not here to do your thinking for you, just to help you think in Perl.

--

Brother Frankus.

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