use strict;
use Regexp::Assemble;

my $re = do {
    open IN, shift || 'file_of_IPs_sought' or die $!;
    my $guts = Regexp::Assemble->new->add(
        map {
            chomp;
            quotemeta($_)
        } <IN>
    )->as_string;
    close IN;
    qr/\b$guts\b/
};

open LOGFILE, shift || 'logfile' or die $!;
/$re/ and print while <LOGFILE>;
close LOGFILE;

# update: if this is a pipe...
/$re/ and print while <>;