There's surely somebody out there, who wrote the code, and could explain it better... ;-)

Aliasing means, that the carcass of the variable is retained (its content saved to be restored later) and filled with a new structure, i.e. it gets a new pointer. Let's have a look. Loop with a symbol table variable:

perl -MDevel::Peek -le 'our $f = 42; Dump *f{SCALAR}; for $f (2) { Dum
+p *f{SCALAR} }'
SV = RV(0x8664670) at 0x863bc28
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x863c7bc
  SV = IV(0x8657eec) at 0x863c7bc
    REFCNT = 2
    FLAGS = (IOK,pIOK)
    IV = 42
SV = RV(0x8664670) at 0x863be2c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x863c7ec
  SV = IV(0x8657ee8) at 0x863c7ec
    REFCNT = 3
    FLAGS = (PADBUSY,PADTMP,IOK,READONLY,pIOK)
    IV = 2
[download]

Note that the scalar variable (SV) contains the same reference value (RV(0x8664670)) outside and inside the loop. It's content however is allocated at a different memory location:

SV = RV(0x8664670) at 0x863bc28 # outside
SV = RV(0x8664670) at 0x863be2c # inside
[download]

That's essentially the same for my variables.

perl -MDevel::Peek -le 'my $f = 42; Dump \$f; for $f (2) { Dump \$f } 
+for my $f (2) { Dump \$f }'
SV = RV(0x9d1a678) at 0x9cf1c28
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x9cf1cdc
  SV = IV(0x9d0def8) at 0x9cf1cdc
    REFCNT = 2
    FLAGS = (PADBUSY,PADMY,IOK,pIOK)
    IV = 42
SV = RV(0x9d1a678) at 0x9cf1e2c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x9cf1c28
  SV = IV(0x9d0defc) at 0x9cf1c28
    REFCNT = 1
    FLAGS = (IOK,pIOK)
    IV = 2
SV = RV(0x9d1a678) at 0x9cf1c28
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x9cf1e2c
  SV = IV(0x9d0defc) at 0x9cf1e2c
    REFCNT = 1
    FLAGS = (IOK,pIOK)
    IV = 2
[download]

Here we have three different storage locations, because the two loops have each its own scope:

SV = RV(0x9d1a678) at 0x9cf1c28
SV = RV(0x9d1a678) at 0x9cf1e2c
SV = RV(0x9d1a678) at 0x9cf1c28
[download]

The RV(0x9d1a678) is the same, which shows us that the following for loops are aliasing the same $f :

my $f;
for $f (1..3) { }
for my $f (1..3) { }
[download]

In the perl sources, you'll find the relevant code which sets up the loop variable at Perl_newFOROP() in file op.c, and the "aliasing code" in pp_ctl.c at PP(pp_enteriter).

update: corrected code locations