I was looking the different times, and saw that Perl-5.8x is about 4 times more slow than Perl-5.6x.

I can be wrong, but on Perl-5.8 UTF-8 will make the strings to alocate 4 bytes for each character. And REGEXP when looking in the string will need to handle that too.

From POD, perlunicode:

UTF-8 is a variable-length (1 to 6 bytes, current character allocation
+s require 4 bytes)...
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And from bytes:

As an example, when Perl sees $x = chr(400), it encodes the character 
+in UTF-8 and stores it in $x. Then it is marked as character data, so
+, for instance, length $x returns 1. However, in the scope of the byt
+es pragma, $x is treated as a series of bytes - the bytes that make u
+p the UTF8 encoding - and length $x returns 2:
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Soo, this code:

$x = chr(400);
print 'Length: ', length $x, qq~\n~;
{ 
  use bytes;
  print 'Length (bytes): ', length $x, qq~\n~;
}
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Has the output:

Length: 1
Length (bytes): 2
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Soo, to see if just a string 4 times bigger can make the REGEXP 4 times slow, make the same test, but adding a string bigger and compare with the tests of this node.

But note that the REGEXP machine in Perl-5.8x is much more complex than in Perl-5.6x just to need to handle the different encode formats that Perl handles. Maybe you need to look for some pragma that disable UTF-8 handling on REGEXP (that I haven't found), and not to try to recompile Perl.