Which perl did you use?

c:\test>perl -v

This is perl, v5.8.6 built for MSWin32-x86-multi-thread
(with 3 registered patches, see perl -V for more detail)

Copyright 1987-2004, Larry Wall

Binary build 811 provided by ActiveState Corp. http://www.ActiveState.
+com
ActiveState is a division of Sophos.
Built Dec 13 2004 09:52:01
[download]

Things do seem to have swung the other way again with 5.10:

for( 30, 300, 3000, 30000 ) { 
    $s = 'x'x$_ . 'reg exp' . 'x'x$_; 
    cmpthese -1, { 
        REGEX => q[ $x = $s =~ /reg exp/;], 
        INDEX => q[ $x = index $s, 'reg exp';] 
    } 
};;
           Rate REGEX INDEX
REGEX 2526899/s    --  -27%
INDEX 3444838/s   36%    --

           Rate REGEX INDEX
REGEX 1083665/s    --   -9%
INDEX 1193837/s   10%    --

          Rate REGEX INDEX
REGEX 159444/s    --   -3%
INDEX 163587/s    3%    --

         Rate REGEX INDEX
REGEX 14881/s    --   -3%
INDEX 15292/s    3%    --

for( 30, 300, 3000, 30000 ) { 
    $s = 'x'x$_ . 'reg exp' . 'x'x$_; 
    study $s; 
    cmpthese -1, { 
        REGEX => q[ $x = $s =~ /reg exp/;], 
        INDEX => q[ $x = index $s, 'reg exp';] 
    }
};;
           Rate REGEX INDEX
REGEX 2103160/s    --  -41%
INDEX 3552278/s   69%    --

           Rate REGEX INDEX
REGEX  806412/s    --  -32%
INDEX 1187789/s   47%    --

          Rate REGEX INDEX
REGEX 110166/s    --  -33%
INDEX 163886/s   49%    --

         Rate REGEX INDEX
REGEX 11587/s    --  -24%
INDEX 15292/s   32%    --
[download]