[14:28:20.34] c:\test>\Perl510\bin\perl5.10.0.exe 688308.pl -loops=1e2
              Rate     reverse substr_copy       subst substr_lval  substr_mod
reverse     5.79/s          --        -78%        -96%        -97%        -98%
substr_copy 26.0/s        349%          --        -84%        -85%        -92%
subst        162/s       2692%        522%          --         -5%        -48%
substr_lval  170/s       2837%        554%          5%          --        -45%
substr_mod   310/s       5257%       1094%         92%         82%          --

[14:29:08.20] c:\test>\Perl510\bin\perl5.10.0.exe 688308.pl -loops=1e3
              Rate     reverse substr_copy       subst substr_lval  substr_mod
reverse     5.91/s          --        -77%        -97%        -97%        -98%
substr_copy 26.1/s        342%          --        -85%        -85%        -92%
subst        169/s       2767%        548%          --         -2%        -47%
substr_lval  173/s       2832%        563%          2%          --        -45%
substr_mod   317/s       5264%       1113%         87%         83%          --

[14:29:44.37] c:\test>\Perl510\bin\perl5.10.0.exe 688308.pl -loops=1e4
              Rate     reverse substr_copy       subst substr_lval  substr_mod
reverse     5.85/s          --        -78%        -96%        -97%        -98%
substr_copy 26.2/s        349%          --        -84%        -85%        -92%
subst        167/s       2750%        535%          --         -5%        -48%
substr_lval  176/s       2908%        570%          6%          --        -45%
substr_mod   321/s       5383%       1121%         92%         82%          --

[14:30:21.73] c:\test>\Perl510\bin\perl5.10.0.exe 688308.pl -loops=1e5
              Rate     reverse substr_copy       subst substr_lval  substr_mod
reverse     5.85/s          --        -78%        -97%        -97%        -98%
substr_copy 26.3/s        350%          --        -84%        -85%        -91%
subst        168/s       2767%        536%          --         -3%        -46%
substr_lval  173/s       2862%        558%          3%          --        -44%
substr_mod   309/s       5191%       1075%         85%         79%          --

##</code><code>##

>\Perl510\bin\perl5.10.0.exe -MDevel::Peek -wle"
    $x = 'fred'; Dump( $x ); 
    print substr( $x, 0, 1 )=''; Dump( $x )"

SV = PV(0x226f64) at 0x182a19c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x18305ec "fred"\0
  CUR = 4
  LEN = 8

SV = PVIV(0x228f40) at 0x182a19c
  REFCNT = 2
  FLAGS = (POK,OOK,pPOK)
  IV = 1  (OFFSET)
  PV = 0x18305ed ( "f" . ) "red"\0
  CUR = 3
  LEN = 7

>\Perl510\bin\perl5.10.0.exe -MDevel::Peek -wle" 
    $x = 'fred'; Dump( $x ); 
    print substr( $x, 0, 1, ''); Dump( $x )"

SV = PV(0x226f64) at 0x182a19c
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x18305ec "fred"\0
  CUR = 4
  LEN = 8
f
SV = PVIV(0x228f40) at 0x182a19c
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  IV = 1  (OFFSET)
  PV = 0x18305ed ( "f" . ) "red"\0
  CUR = 3
  LEN = 7