>perl -wMstrict -le
"my %replace = (
   DEXX  => 'AREX',
   AREX  => 'CUBE',
   ABC   => 'VWX',
   ABCD  => 'VWXY',
   ABCDE => 'VWXYZ',
   );
 my $find = join '|', map quotemeta, keys %replace;
    $find = qr{ $find }xms;
 print qq{find regex: $find};
 my $s = 'DEXX AREX CUBE ABC ABCD ABCDE';
 print qq{before: '$s'};
 (my $t = $s) =~ s{ ($find) }{$1/$replace{$1}}xmsg;
 print qq{after:  '$t'};
 print '';
 my $longest = join '|', map quotemeta, reverse sort keys %replace;
    $longest = qr{ $longest }xms;
 print qq{find regex (longest match): $longest};
 print qq{before: '$s'};
 ($t = $s) =~ s{ ($longest) }{$1/$replace{$1}}xmsg;
 print qq{after:  '$t'};
"
find regex: (?msx-i: DEXX|ABC|ABCD|ABCDE|AREX )
before: 'DEXX AREX CUBE ABC ABCD ABCDE'
after:  'DEXX/AREX AREX/CUBE CUBE ABC/VWX ABC/VWXD ABC/VWXDE'

find regex (longest match): (?msx-i: DEXX|AREX|ABCDE|ABCD|ABC )
before: 'DEXX AREX CUBE ABC ABCD ABCDE'
after:  'DEXX/AREX AREX/CUBE CUBE ABC/VWX ABCD/VWXY ABCDE/VWXYZ'

##</code><code>##

>perl -wMstrict -le
"my %replace = (
   ABC   => 'XXX',
   ABCD  => 'YYYY',
   ABCDE => 'ZZZZZ',
   );
 my $find = join '|', map quotemeta, reverse sort keys %replace;
    $find = qr{ $find }xms;
 print qq{find regex: $find};
 my $s = 'ABC ABCD xxABCDxx ABCDE';
 print qq{before: '$s'};
 (my $t = $s) =~ s{ ($find) }{$replace{$1}}xmsg;
 print qq{sans \\b: '$t'};
 print '';
 print qq{before: '$s'};
 ($t = $s) =~ s{ \b ($find) \b }{$replace{$1}}xmsg;
 print qq{with \\b: '$t'};
"
find regex: (?msx-i: ABCDE|ABCD|ABC )
before:  'ABC ABCD xxABCDxx ABCDE'
sans \b: 'XXX YYYY xxYYYYxx ZZZZZ'

before:  'ABC ABCD xxABCDxx ABCDE'
with \b: 'XXX YYYY xxABCDxx ZZZZZ'