$_ = "abcde";
if (/(.)(.)/) { # first two
  if (/(.)(.)$/) { # last two, by your theory won't upset
    print "inner: $1 $2\n";
  }
  # by your theory, should see first two again
  print "outer: $1 $2\n";
}

##</code><code>##

inner: d e
outer: d e

##</code><code>##

$_ = "abcde";
if (/(.)(.)/) { # first two
  { # ADDED
  if (/(.)(.)$/) { # last two, by your theory won't upset
    print "inner: $1 $2\n";
  }
  } # ADDED
  # by your theory, should see first two again
  print "outer: $1 $2\n";
}

##</code><code>##

inner: d e
outer: a b