use strict;
use warnings;

my %d = (
    foo => sub { "[@_]"; },
    bar => sub { "(@_)"; },
);

$_ = 'baz1';
s/(\w+)(??{!$d{$1}})/$d{$1}->($1)/ge; 
print "$_\n";
[download]

Use of uninitialized value in subroutine entry at line 10.
Can't use string ("") as a subroutine ref while "strict refs" in use a
+t line 10.
[download]

$1 is is "baz", so $d{$1} is undef, so !$d{$1} is 1, so the match tries to match "1". It does, so the replacement expression is executed.