Here is another way to do it. As we have the values in a hash we can easily see if we also have $value+1. It does not come close to the elegance of the regex linked above. sub bar is a drop in replacement for sub foo.

sub bar {
    my %hash = @_;
    foreach (keys %hash) {
        if (exists $hash{$_+1}) {
            $hash{$_}="."
        }else{
            $hash{$_}=""
        }
    }
    my @array = ( sort {$a <=> $b}  (keys %hash) );
    my $string = shift @array;
    $string.=$hash{$string};
    foreach (@array) {
        $string .= ", $_$hash{$_}";
    }
    $string=~s/(, (\d+)\.)(, (\d+)\.)*/, $2../g;
    $string=~s/\.\., /../g;
    print "\n($string)\n";
}

# updated ! this is a nicer way to print the output

sub bar {
    my %hash = @_;
    foreach (keys %hash) {
        if (exists $hash{$_+1}) {
            $hash{$_}="."
        }else{
            $hash{$_}=""
        }
    }
    my @array = ( sort {$a <=> $b}  (keys %hash) );
    my $last=pop @array;
    my $lastdot=0;
    print "\n(";
    foreach (@array) {
        if ($hash{$_}) {
            next if $lastdot;
            print "$_..";
            $lastdot++;
        } else {
            print "$_, ";
            $lastdot=0;
        }
    }
    print "$last)\n";
}
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Cheers,
R.