You can try the following code I quickly wrote. It's just a quick and simple hack. If you want to have command-line options, then you need to use the Getopt::Long module from CPAN.

use IO::File;
use strict;

# Load host list
my $host = new IO::File "host.txt", "r"
    or die "Can not open host list file!";
my @hosts = <$host>;
undef $host;
foreach (@hosts) {
    chomp;
    s/\s+//g;   # get rid of spaces
}

# Load input lines
my $input = new IO::File "mainfile.txt", "r"
    or die "Can not open file!";
my @lines = <$input>;
undef $input;
foreach (@lines) { chomp };

# Create output files for hosts
my $prefix = "mainfile_";
foreach (@hosts)
{
    my ($hostname, $ipaddr) = split/,/;
    
    # Create the output file, and replace the
    # HOST and IP lines with actual data
    my $output = new IO::File "${prefix}${hostname}.txt", "w"
        or die "Can not create file!";
        
    foreach my $txt (@lines)
    {
        if ($txt eq "HOST") {
            print $output "$hostname\n";
        } elsif ($txt eq "IP") {
            print $output "$ipaddr\n";
        } else {
            print $output "$txt\n";
        }           
    }
    
    # explicitly close the output file
    # not required, but good for the eye
    undef $output;
}
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