It does modify the elements because the elements are not copied to the list, instead, they are referenced there:

use Devel::Peek;

my @array = (1,2,3);
Dump \@array;

print "====\n";

foreach (@array) {
    Dump $_;
    print "----\n";
}
[download]

Will output:

SV = RV(0x193de54) at 0x22593c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x226680
  SV = PVAV(0x22a95c) at 0x226680
    REFCNT = 2
    FLAGS = (PADBUSY,PADMY)
    IV = 0
    NV = 0
    ARRAY = 0x19240d4
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x1926b90) at 0x225858
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 1
    Elt No. 1
    SV = IV(0x1926b94) at 0x225918
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 2
    Elt No. 2
    SV = IV(0x1926b98) at 0x225930
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 3
====
SV = IV(0x1926b90) at 0x225858
  REFCNT = 2
  FLAGS = (IOK,pIOK)
  IV = 1
----
SV = IV(0x1926b94) at 0x225918
  REFCNT = 2
  FLAGS = (IOK,pIOK)
  IV = 2
----
SV = IV(0x1926b98) at 0x225930
  REFCNT = 2
  FLAGS = (IOK,pIOK)
  IV = 3
----
[download]

As you can see, the IVs printed in the for loop are the same ones from the array (same pointer value) but they now have a reference count REFCNT=2

This means that the IVs were referenced in the new list, so that's why modifying them there will also modify them in the array.

--
Leviathan.