use strict;

my @ips =
(
    "192.1.20.1",
    "192.1.20.2",
);

my %ips = map { $_=>1 } @ips;

open LOG, "<", "logfile" or die $!;
while ( <LOG> )
{
    #match ip-address
    if ( /(([0-9]+\.)+[0-9]+)/ )
    {
        if ( $ips{$1} )
        {
            # do found ip stuff here
        }
        else
        {
            #do other stuff here
        }
    }
}
close LOG;