fix: preserve errno across CORE::GLOBAL success paths

lib/Test/MockFile.pm

@@ -1929,6 +1929,9 @@ sub _abs_path_to_file {
 
     return unless defined $path;
 
+    # Protect caller's $! — internal calls (getcwd, getpwent) may clobber errno
+    local $!;
+
     # Tilde expansion must happen before making the path absolute
     # ~
     # ~/...
@@ -1985,7 +1988,8 @@ sub __cwd_abs_path {
 
     # Make absolute without collapsing .. (symlink-aware resolution does that)
     if ( $path !~ m{^/} ) {
-        $path = Cwd::getcwd() . "/$path";
+        my $cwd = do { local $!; Cwd::getcwd() };
+        $path = $cwd . "/$path";
     }
 
     my @remaining = grep { $_ ne '' && $_ ne '.' } split( m{/}, $path );

t/errno_preservation.t

@@ -0,0 +1,86 @@
+#!/usr/bin/perl -w
+
+use strict;
+use warnings;
+
+use Test2::Bundle::Extended;
+use Test2::Tools::Explain;
+use Test2::Plugin::NoWarnings;
+
+use Errno qw( ENOENT EACCES );
+use Test::MockFile qw< nostrict >;
+use Cwd ();
+
+# ==========================================================================
+# GH #396: CORE::GLOBAL overrides must not clobber $! on success paths
+# ==========================================================================
+
+# _abs_path_to_file calls getcwd() for relative paths and getpwent() for
+# tilde paths. Both can set $! as a side effect. The fix wraps these in
+# local $! so callers see the errno they set, not internal noise.
+
+# --------------------------------------------------------------------------
+# open() on absolute path should not clobber $!
+# --------------------------------------------------------------------------
+{
+    my $mock = Test::MockFile->file( '/tmp/errno_test.txt', 'data' );
+
+    $! = ENOENT;    # Set a known errno
+    open( my $fh, '<', '/tmp/errno_test.txt' ) or die "open failed: $!";
+    is( $! + 0, ENOENT, 'open (absolute path) preserves $! on success' );
+    close $fh;
+}
+
+# --------------------------------------------------------------------------
+# stat() on absolute path should not clobber $!
+# --------------------------------------------------------------------------
+{
+    my $mock = Test::MockFile->file( '/tmp/errno_stat.txt', 'data' );
+
+    $! = EACCES;
+    my @st = stat('/tmp/errno_stat.txt');
+    ok( @st > 0, 'stat succeeds on mocked file' );
+    is( $! + 0, EACCES, 'stat (absolute path) preserves $! on success' );
+}
+
+# --------------------------------------------------------------------------
+# unlink() should not clobber $! on success
+# --------------------------------------------------------------------------
+{
+    my $mock = Test::MockFile->file( '/tmp/errno_unlink.txt', 'data' );
+
+    $! = ENOENT;
+    my $ret = unlink('/tmp/errno_unlink.txt');
+    is( $ret, 1, 'unlink succeeds' );
+    is( $! + 0, ENOENT, 'unlink preserves $! on success' );
+}
+
+# --------------------------------------------------------------------------
+# Cwd::abs_path override should not clobber $! on success
+# --------------------------------------------------------------------------
+{
+    my $mock = Test::MockFile->file( '/tmp/errno_cwd.txt', 'data' );
+
+    $! = EACCES;
+    my $abs = Cwd::abs_path('/tmp/errno_cwd.txt');
+    is( $abs, '/tmp/errno_cwd.txt', 'abs_path resolves correctly' );
+    is( $! + 0, EACCES, 'Cwd::abs_path preserves $! on success' );
+}
+
+# --------------------------------------------------------------------------
+# readdir should not clobber $! on success
+# --------------------------------------------------------------------------
+{
+    my $mock_dir = Test::MockFile->dir('/tmp/errno_dir');
+    my $mock_f   = Test::MockFile->file( '/tmp/errno_dir/a.txt', 'hello' );
+
+    $! = ENOENT;
+    opendir( my $dh, '/tmp/errno_dir' ) or die "opendir: $!";
+    # opendir may legitimately change $! — reset after
+    $! = ENOENT;
+    my @entries = readdir($dh);
+    ok( @entries > 0, 'readdir returns entries' );
+    closedir($dh);
+}
+
+done_testing;