sub foo {
     my($qr, $replace) = @_;
     s/$qr/$replace->()/ge;
}

$_ = "This is a new dawn.";
foo(qr/(\w+)/, sub { ucfirst $1 });
print;

##</code><code>##

sub bar {
   my $callback = shift;
   $callback->();
}
bar(sub { s/(\w+)/\u$1/g; });