use strict;
use warnings;

my $file = 'input.txt';

my (%hash, @ips, @alerts);

open (FILE, "$file") or die "Can't open $file\n";

while (<FILE>){

  $/ = '-------------------------------------------------';

  $hash{$2}{1} = 0 if (/(Host: \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}).*
                       (Service:.*Severity:.*)/ms);

}

foreach my $key (keys %hash) {
    $hash{$key} = [keys %{$hash{$key}}];
}