fix: preserve localized input line counter changes

docs/about/changelog.md

@@ -26,6 +26,8 @@ priorities and future plans.
 
 - Preserve the input line number when a borrowed anonymous filehandle alias leaves a JVM subroutine scope (#1933).
 
+- Preserve filehandle line-counter assignments made through localized `$.` scopes (#1965).
+
 - 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/backend/bytecode/BytecodeInterpreter.java

@@ -81,6 +81,10 @@ static boolean isImmutableProxy(RuntimeBase val) {
         // interpreter's mutating opcodes must NOT silently strip it --
         // we want `for (3) { ++$_ }` to throw, not silently succeed.
         if (val instanceof ReadOnlyAlias) return false;
+        if (val instanceof ScalarSpecialVariable specialVariable
+                && specialVariable.hasSpecialAssignmentBehavior()) {
+            return false;
+        }
         return val instanceof RuntimeScalarReadOnly || val instanceof ScalarSpecialVariable;
     }
 

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

@@ -64,6 +64,14 @@ public boolean isNumberedCapture() {
         return variableId == Id.CAPTURE;
     }
 
+    /** Whether assignments must reach this special scalar's setter. */
+    public boolean hasSpecialAssignmentBehavior() {
+        return switch (variableId) {
+            case INPUT_LINE_NUMBER, HINTS, WARNING_BITS, REMOVED_ENCODING -> true;
+            default -> false;
+        };
+    }
+
     /**
      * Throws an exception as this variable represents a constant item
      * and cannot be modified.
@@ -530,9 +538,10 @@ public void dynamicRestoreState() {
             if (!inputLineStateStack.isEmpty()) {
                 InputLineState previous = inputLineStateStack.pop();
                 RuntimeIO.setLastAccessedHandle(previous.lastHandle);
-                if (previous.lastHandle != null) {
-                    previous.lastHandle.currentLineNumber = previous.lastLineNumber;
-                }
+                // $. localization restores which handle is current, but the
+                // line counter belongs to that handle. Assignments to $. (and
+                // reads from the handle) change its counter permanently, even
+                // when they happen inside a dynamically localized scope.
                 lvalue = previous.localValue;
                 // Do not sync this.type/value from lvalue here; $. reads always
                 // delegate to currentLineNumber via getValueAsScalar().

src/test/resources/unit/localized_input_line_number.t

@@ -0,0 +1,32 @@
+use strict;
+use warnings;
+use Test::More;
+
+my $contents = "one\ntwo\nthree\n";
+open my $fh, '<', \$contents or die "open scalar handle: $!";
+
+<$fh>;
+<$fh>;
+is($., 2, 'reading updates the current handle line number');
+
+seek($fh, 0, 0) or die "seek scalar handle: $!";
+if (1) {
+    local $. = 0;
+}
+is($., 0, 'localized assignment to $. remains on the filehandle after scope exit');
+is(scalar <$fh>, "one\n", 'the handle was rewound');
+is($., 1, 'reading after the rewind starts at line one');
+
+my $other_contents = "other one\nother two\n";
+open my $other, '<', \$other_contents or die "open second scalar handle: $!";
+<$fh>;
+<$fh>;
+is($., 3, 'the original handle has advanced to line three');
+{
+    local $.;
+    <$other>;
+    is($., 1, 'reading another handle updates $. inside the localized scope');
+}
+is($., 3, 'scope exit restores the previously current handle');
+
+done_testing;