perl -MO=Concise -e 'for(1 .. 100){print if $_ < 50}'
perl -MO=Concise -e 'for(grep $_ < 50, (1 .. 100)){print}'

You'll notice that you've just traded an "and" and a few constants for that larger grep.
     leaveloop
       enteriter
          pushmark
-         const
-         const
+         grepwhile
+            grepstart
+               pushmark
+               lt
+                  gvsv
+                  const
+               rv2av
+                  const
          gv
          and
             iter
             lineseq
                nextstate
-               and
-                  lt
-                     gvsv
-                     const
-                  print
-                     pushmark
-                     gvsv
+               print
+                  pushmark
+                  gvsv
                unstack
                nextstate

perl -MO=Concise -e '$array_ref=[1..10000];for(@$array_ref){print}'
perl -MO=Concise -e '$array_ref=[1..10000];for($i=0;$_=$array_ref->[$i];$i++){print}'

Here you'll notice that you you doubled the number of operations being performed for handling the first construct. Ugly.
    nextstate
-   leaveloop
-      enteriter
-         pushmark
-         rv2av
-            rv2sv
-               gv
-         gv
-         and
-            iter
-            lineseq
-               nextstate
-               print
-                  pushmark
+   sassign
+      const
+      gvsv
+   lineseq
+      nextstate
+      leaveloop
+         enterloop
+            and
+               sassign
+                  aelem
+                     rv2av
+                        rv2sv
+                           gv
+                     gvsv
                   gvsv
-               unstack
-               nextstate
+               lineseq
+                  scope
+                     print
+                        pushmark
+                        gvsv
+                  preinc
+                     gvsv
+                  unstack
+                  nextstate