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%d = (
    foo => sub { "[@_]"; },
...
$_ = 'foo fooz bar baz1';
s/(\w+)(?(?{ !$d{$1} })(?!))/$d{$1}->($1)/ge;
print "$_\n";

[foo] [foo]z (bar) baz1

s/
    (\w+)   # One or more word chars in capture buffer 1. Got it.
...
# 3. (?!) says to *not* match a "1" followed by null. Maybe?
# 4. (?...) means use the 1 not followed by null as a regexp modifier
#    to 'foo'? What?!