I guess that this would also be a solution.
Adapt to your application..

#!/usr/bin/perl -w
use strict;

my $heredoc = <<END;
This is a whole bunch of stuff with Jupiter
and Mars and Earth and Venus and Saturn.
Also could be Earth and Earth and something else like Neptune.
END

my $xlatedoc = <<END;
Jupiter host1
Mars    host2
Earth   host3
END

my %xlate;

open (XLATE, '<', \$xlatedoc) or die "xlatedoc failed: $!\n";
while (<XLATE>)
{
   my ($planet, $host) = split; #no chomp() needed 
   $xlate{$planet}=$host;
}

close (XLATE);

my @planets = keys %xlate;
my $all_planets = join ('|',@planets);

open (IN, '<', \$heredoc) or die "heredoc failed: $!\n";
while (<IN>)
{
   s/($all_planets)/$xlate{$1}/g;
     # $all_planets is an OR expression
     # $1 winds up being which one of the OR'ed terms matched
     # that gets translated to the new term
   print;
}

__END__
This is a whole bunch of stuff with host1
and host2 and host3 and Venus and Saturn.
Also could be host3 and host3 and something else like Neptune.
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