diff --git a/UPGRADING.md b/UPGRADING.md
index a682da0..eb168d6 100644
--- a/UPGRADING.md
+++ b/UPGRADING.md
@@ -1,3 +1,19 @@
+# (2023-08-21) RevBank 3.9
+
+A tiny change that could break things: the shebang was changed from
+`#!/usr/bin/perl` to the more modern `#!/usr/bin/env perl`.
+
+In the unlikely event that your system has multiple perl executables in the
+search path of `PATH`, this change could mean that revbank now uses a different
+perl, in which case you may have to reinstall the required Perl libraries.
+
+Background: NixOS doesn't follow the previously uni(x)versal convention that,
+when Perl is available, an executable exists at `/usr/bin/perl`. The only
+stable paths that NixOS provides for shebangs are `#!/bin/sh` or
+`#!/usr/bin/env`. There were always pros and cons to switching the shebang to
+`env` (e.g. for use with perlbrew), but learning about Nix has tipped the
+scales for me. (The performance penalty is not relevant for RevBank.)
+
 # (2023-02-12) RevBank 3.8
 
 ## Update your `revbank.plugins`
diff --git a/revbank b/revbank
index ff910b6..2634ff1 100755
--- a/revbank
+++ b/revbank
@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 use v5.28;
 use warnings;
@@ -18,7 +18,7 @@ use RevBank::Global;
 use RevBank::Messages;
 use RevBank::Cart;
 
-our $VERSION = "3.8";
+our $VERSION = "3.9";
 our %HELP1 = (
     "abort" => "Abort the current transaction",
 );