$string = "We modify foos and bars but not foobars.\n";

my %trans = qw(
  foo     bar
  bar     foo
  foobar  foobar
);

# reverse sort is a hack to put strings before
# substrings.  (ie foobar before foo)
my $re_str =  join "|", map quotemeta($_),
   reverse sort keys %trans;
$string =~ s/($re_str)/$trans{$1}/g;
print $string;