my $file = "$file_path/file.lst";
my $newid = "500.0.0.0"
open MAST, $file or die "Unable to open file.lst: $!";
my $new = "$file.tmp.$$";
my $bak = "$file.bak";
open(NEW, "> $new")         or die "can't open $new: $!";

while (<MAST>) {
my ($pattern,$id) = (split /\s+/, $_)[0,4];
        print $_;
        if ( $_ =~ m/^$pattern/ ) {
             $_ =~ s/$id/$newid/g;
        }
        (print NEW $_)          or die "can't write to $new: $!";
}

close(MAST)                    or die "can't close $file: $!";
close(NEW)                     or die "can't close $new: $!";

rename($file, $bak)          or die "can't rename $file to $bak: $!";
rename($new, $file)          or die "can't rename $new to $file: $!";