First off, you don't need a fancy sort subroutine if all you're using is regular numbers. The famous "spaceship operator" (<=>) will do quite nicely.(see below.)

#!/usr/bin/perl -w

use strict;

# example input string
my $in = '1089,3,4,5,6,7,99,832,1087,831,1088';

# split the scalar
my @i = split(/,/,$in);

# to sort by number just use $a <=> $b instead 
# of the default $a cmp $b
@i = sort {$a <=> $b} @i;

# load a hash to make looking up existing numbers easy
my %lookup;
foreach my $number(@i) {
    $lookup{$number} = 1;
}

# now check the numbers for consecutiveness, and
# put the results in @output
my @output;
my $end_number=0;

foreach my $number (@i) {
    # skip the numbers already included in a range
    next if $number <= $end_number;
    
    # find the end_number in a range
    $end_number = $number;
    $end_number++ until !$lookup{$end_number+1};
    
    # and add to @output whether the range has
    # one or more numbers in it
    if ($number == $end_number) {
        push @output, $number;
    } else {
        push @output, "$number-$end_number";
    }
}

print "$_\n" for @output;
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There's lots more fancy ways of doing this. The code above is optimized for read-ability and learn-ability.

(and, for the record, yes, it looks a little homework-ey, but anyone who posts code has at least given it a shot.)

Gary Blackburn
Trained Killer