c:\@Work\Perl>perl -le
"use 5.010;
 ;;
 use warnings;
 use strict;
 ;;
 my %direct_substitution = (
   'apple' => 'PEAR',
   'red'   => 'YELLOW',
   'xyzzy' => 'SOME OTHER THING',
   );
 ;;
 my ($search) =
   map  qr{ \b (?: $_ ) \b }xms,
   join q{ | },
   keys %direct_substitution
   ;
 print $search;
 ;;
 my @lines = (
   'apple xapple apple applex xapplex apple',
   'red',
   'xxx red apple yyyy xyzzy zz',
   );
 ;;
 my $count = 0;
 $count += s{ ($search) }{$direct_substitution{$1}}xmsg for @lines;
 ;;
 print qq{substitutions: $count};
 print qq{'$_'} for @lines;
"
(?^msx: \b (?: apple | red | xyzzy ) \b )
substitutions: 7
'PEAR xapple PEAR applex xapplex PEAR'
'YELLOW'
'xxx YELLOW PEAR yyyy SOME OTHER THING zz'