If you're concerned about memory usage, you shouldn't read the entire file at once. Read it line by line and count the IPs in a hash, so you don't get duplicated entries. And, of course, use strict, but I guess you knew that already :) Anyhow, here's a snipped that came to my mind:

#!/usr/bin/perl -w

use strict;

my $subnet = '192.168.87';
my %data = ();
# precompile regex for performance..
my $regex = qr#^($subnet\.\d+)#;

# read file line by line
while (my $line = <DATA>)
{
    chomp($line);

    if ($line =~ $regex)
    {
            $data{$1}++;
    }
    elsif ($debug)
    {
            print STDERR "Didn't match: $line\n";
    }
}
[download]

If you really do want to stick to your own code, your line is better written as (it's not very nice either..):

my @matches = grep { m/$regex/ } @lines;
($_) = m/$regex/ foreach @matches;
[download]

Regards,
-octo-