>perl -wMstrict -le
"my %d = (
     foo => sub { \"[@_]\"; },
     bar => sub { \"(@_)\"; },
 );
 $_ = 'foo fooz bar';
 s/ (\w+) (?(?{ !$d{$1} }) (?!)) / $d{$1}->($1) /xge;
 print;
"
[foo] [foo]z (bar)

>perl -wMstrict -le
"my %d = (
     foo => sub { \"[@_]\"; },
     bar => sub { \"(@_)\"; },
 );
 $_ = 'foo fooz bar';
 s/ (\w+) / $d{$1}->($1) /xge;
 print;
"
Use of uninitialized value in subroutine entry at ...
Can't use string ("") as a subroutine ref while "strict refs" ...

>perl -wMstrict -le
"my %d = (
     foo => sub { \"[@_]\"; },
     bar => sub { \"(@_)\"; },
 );
 $_ = 'foo fooz bar';
 s/ (\w+) / $d{$1} ? $d{$1}->($1) : $1 /xge;
 print;
"
[foo] fooz (bar)