diff -up perl-5.12.3/pp.c.87 perl-5.12.3/pp.c
--- perl-5.12.3/pp.c.87	2011-01-09 21:20:58.000000000 +0100
+++ perl-5.12.3/pp.c	2011-04-01 15:06:23.000000000 +0200
@@ -3949,6 +3949,8 @@ PP(pp_ucfirst)
 	    SvCUR_set(dest, need - 1);
 	}
     }
+    if (dest != source && SvTAINTED(source))
+        SvTAINT(dest);
     SvSETMAGIC(dest);
     RETURN;
 }
@@ -4008,7 +4010,8 @@ PP(pp_uc)
 	SvUPGRADE(dest, SVt_PV);
 	d = (U8*)SvGROW(dest, min);
 	(void)SvPOK_only(dest);
-
+	if (dest != source && SvTAINTED(source))
+	    SvTAINT(dest);
 	SETs(dest);
     }
 
@@ -4433,6 +4436,8 @@ PP(pp_lc)
 	    SvCUR_set(dest, d - (U8*)SvPVX_const(dest));
 	}
     }
+    if (dest != source && SvTAINTED(source))
+        SvTAINT(dest);
     SvSETMAGIC(dest);
     RETURN;
 }
diff -up perl-5.12.3/t/op/taint.t.87 perl-5.12.3/t/op/taint.t
--- perl-5.12.3/t/op/taint.t.87	2011-01-09 21:20:58.000000000 +0100
+++ perl-5.12.3/t/op/taint.t	2011-04-01 15:07:43.000000000 +0200
@@ -17,7 +17,7 @@ use Config;
 use File::Spec::Functions;
 
 BEGIN { require './test.pl'; }
-plan tests => 302;
+plan tests => 306;
 
 $| = 1;
 
@@ -1318,6 +1318,18 @@ foreach my $ord (78, 163, 256) {
     unlike($err, qr/^\d+$/, 'tainted $!');
 }
 
+{
+    # [perl #87336] lc/uc(first) failing to taint the returned string
+    my $source = "foo$TAINT";
+    my $dest = lc $source;
+    test $dest, "lc(tainted) taints its return value";
+    $dest = lcfirst $source;
+    test $dest, "lcfirst(tainted) taints its return value";
+    $dest = uc $source;
+    test $dest, "uc(tainted) taints its return value";
+    $dest = ucfirst $source;
+    test $dest, "ucfirst(tainted) taints its return value";
+}
 
 # This may bomb out with the alarm signal so keep it last
 SKIP: {