#!/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";
    }
}

##</code><code>##

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