aaron_baugher provided a good solution. Nevertheless, here's a longer (and not necessarily prettier nor essentially operationally different) option. It uses only one old/new file, where the old/new word pairs are tab delimited, e.g.:

Earth    Host1
Jupiter    Host2
Venus    Host3
Mars    Host4
[download]

It uses File::Slurp for file read/write operations. The regex matches the old words on word boundaries, in case you don't want to replace embedded 'words:'

use Modern::Perl;
use File::Slurp qw/read_file write_file/;

my $logFile = 'log.txt';
my $oldNewFile = 'oldNewFile.txt';

my $text = read_file $logFile;

for(read_file $oldNewFile){
    my ($old, $new) = split;
    $text =~ s/\b$old\b/$new/g
}

say $text;
#write_file( $logFile, $text );
[download]

If you're satisfied with the replacement results shown in the printed output, you can uncomment the #write_file... line to save the changes to log.txt. I recommend running it on a test file, first.

Hope this helps!