use warnings;
use strict;

my %words = (word => 1, word2 => 1, word3 =>1);
my $temp  = 'WORD2';
my $temp1 = 'word2';
$_ = 'here is word2';
s/$temp1/$words{$temp1} ? "$temp " : "[$temp] "/ex; 
print "$_\n";

__END__

here is WORD2