Don't use a regex. Parse the file line by line.

my %leases;
my $curr_lease;
while (<FH>)
{
    # We're done with the current lease
    if (/}/)
    {
        $curr_lease = '';
    }

    elsif (/^lease (\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})/
    {
        $current_lease = $1;
        $leases{$current_lease} = {};
    }

    elsif (/^\s+(starts|ends) (\d) (\d(4)\/\d{2}\/\d{2}) (\d{2}:\d{2}:
+\d{2})/)
    {
        $leases{$current_lease}{$1} = {
            num  => $2,
            date => $3,
            time => $4,
        };
    }

    elsif (/^\s+(hardware) (\w+) ([^;]+)/)
    {
        $leases{$current_lease}{$1} = {
            type => $2,
            id => $3,
        };
    }

    elsif ((my @vals = split) == 2)
    {
        $leases{$current_lease}{$vals[0]} = $vals[1];
    }

    else
    {
        chomp;
        print "I don't know what to do with '$_'\n";
    }
}
[download]

Now, you have all the leases in %leases, keyed by IP number. :-)

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.