use strict;
use warnings;

my $file = 'input.txt';

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

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

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}}];
}

my ($key, $values);
while (($key, $values) = each %hash) {
    print "$key --> ", join('|', @$values), "\n";
}