my %hashstore = ( "DEXX" => "AREX", "AREX" => "CUBE" );

my $doc1 = "DEXX";
my $doc2 = "DEXX";
my $doc3 = "DEXX";

#foreach (keys %hashstore){
foreach ("DEXX", "AREX"){ # Make sure we get them in the worse order.
    $doc1=~s#\b($_)\b#$1/$hashstore{uc($_)}#ig;
}

#foreach (keys %hashstore){
foreach ("DEXX", "AREX"){ # Make sure we get them in the worse order.
    $doc2=~s#(?<!/)\b($_)\b#$1/$hashstore{uc($_)}#ig;
}

my $pat = join '|', keys(%hashstore);
$doc3 =~ s!\b($pat)\b!$1/$hashstore{uc($1)}!ig;

print("$doc1\n");  # DEXX/AREX/CUBE
print("$doc2\n");  # DEXX/AREX
print("$doc3\n");  # DEXX/AREX