# set_default_host.pl
use strict;
use Regexp::Common qw /net/;

if (scalar @ARGV == 1 and $ARGV[0]=~/^$RE{net}{IPv4}$/) {

    my $path2hostsfile = 'C:/Windows/System32/drivers/etc/hosts';
    my @hostsfile;

    {
        open(my $from_hostsfile, '<', $path2hostsfile) or die "Could not open $path2hostsfile for reading";
        @hostsfile = <$from_hostsfile>;
    }

    foreach my $line (@hostsfile) {
        next unless $line =~/^\s*$RE{net}{IPv4}\s+legba/; # skip unless it is the default entry
        $line = "$ARGV[0]    legba\n";  # change the default host. $line is an alias for the corresponding element of @hostsfile!
    }

    {
        open(my $to_hostsfile,   '>', $path2hostsfile) or die "Could not open $path2hostsfile for writing";
        print $to_hostsfile @hostsfile;
    }
}
else {
    print "Missing or wrong argument!\nusage: set_default_host.pl default_IP\n";
}