fix: preserve $. across borrowed handle scope cleanup

docs/about/changelog.md

@@ -6,6 +6,8 @@ priorities and future plans.
 
 ## Work in progress
 
+- Preserve the input line number when a borrowed anonymous filehandle alias leaves a JVM subroutine scope (#1933).
+
 - Resolve glob-backed filehandles in `POSIX::isatty` without assuming the glob value is a `RuntimeIO` (#1932).
 
 - Add an initial CryptX Java XS bridge for binary/base64url helpers, functional digests, and HMAC using the existing JCA and Bouncy Castle providers (#1220).

src/main/java/org/perlonjava/runtime/runtimetypes/RuntimeScalar.java

@@ -5201,8 +5201,16 @@ public static void scopeExitCleanup(RuntimeScalar scalar) {
             RuntimeIO last = RuntimeIO.getLastAccessedHandle();
             if ((glob.globName != null && glob.globName.equals(last.globName))
                     || (glob.IO != null && glob.IO.value == last)) {
-                RuntimeIO.setLastAccessedHandle(null);
-                RuntimeIO.setLastReadlineHandleName(null);
+                // A borrowed anonymous handle alias can leave scope while the
+                // caller still owns the same IO glob. Keep its $. context until
+                // that durable owner leaves; clearing on the borrow breaks
+                // getline/input_line_number calls inside wrapper subroutines.
+                boolean borrowedLiveIo = isUnstashedIoGlob(glob)
+                        && !scalar.ioOwner && glob.ioHolderCount > 0;
+                if (!borrowedLiveIo) {
+                    RuntimeIO.setLastAccessedHandle(null);
+                    RuntimeIO.setLastReadlineHandleName(null);
+                }
             }
         }
 

src/test/resources/unit/io_handle_input_line_number.t

@@ -0,0 +1,21 @@
+use strict;
+use warnings;
+use Test::More;
+use IO::Handle;
+
+my $input = "first\nsecond\n";
+open my $fh, '<', \$input or die "open scalar handle: $!";
+
+is($fh->getline, "first\n", 'read the first line from a scalar handle');
+is($., 1, 'the global input line number follows getline');
+my $read_line_number = sub { $. };
+is($read_line_number->(), 1,
+    'the global input line number is visible inside a subroutine');
+is($fh->input_line_number, 1,
+    'input_line_number reports the current line after getline');
+
+is($fh->getline, "second\n", 'read the second line from a scalar handle');
+is($fh->input_line_number, 2,
+    'input_line_number advances with subsequent getline calls');
+
+done_testing;