Explain what you're trying to do. I think you're looking for Perl's special variables and not the escape sequences. For example, if you change $\, you change how Perl defines the end of a line when reading a file.

As for changing character classes ... I would strongly advise against that. Instead, I would define your own character classes as so:

my $class = qr/[a-yA-Y]/; # Don't want to match z or Z
if ($string =~ /^${class}+$/)
{
    print "'$string' doesn't have a z in it!\n";
}
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Similar arguments can be made for the other regex escape sequences. Remember - if you redefine it for the entire of Perl, you have redefined it for all the modules you use, such as DBI, CGI, etc. I don't think that's what you really want to do.