Nice, but after some playing around I can guarantee that you have solved the least of my problems.

What I really want is conditional support for $` and $'. That is much worse.

To give a simple example, try the following 3 programs:

#!/usr/bin/perl
# This demonstrates matching through a string
use strict;
use Time::HiRes qw(gettimeofday);
my $start = gettimeofday();
my $str = "_" x $ARGV[0];
1 while $str =~ /./g;
my $elapsed = gettimeofday() - $start;
print "$ARGV[0] characters took $elapsed seconds\n";

#!/usr/bin/perl
# This demonstrates matching through a string, capturing
use strict;
use Time::HiRes qw(gettimeofday);
my $start = gettimeofday();
my $str = "_" x $ARGV[0];
1 while $str =~ /(.)/g;
my $elapsed = gettimeofday() - $start;
print "$ARGV[0] characters took $elapsed seconds\n";

#!/usr/bin/perl
# This demonstrates matching through a string, capturing $`
use strict;
use Time::HiRes qw(gettimeofday);
# Mess life up here
if ("gotcha" =~ /o/) {
  my $fooey = $`;
}
my $start = gettimeofday();
my $str = "_" x $ARGV[0];
1 while $str =~ /(.)/g;
my $elapsed = gettimeofday() - $start;
print "$ARGV[0] characters took $elapsed seconds\n";
[download]

If you try it you will find that the first two versions run linearly, with only a modest speed difference for the capturing. But the third is a quadratic speed drop. Should you ever, as I do, use REs as a way of tokenizing, this means that the use of a single $` or $', anywhere, turns linear algorithms quadratic. By contrast turning $& on and off is not going to change your program's scalability.

For that reason if I write something for other people's use, I would really like the option of turning off $` and $' lexically. Because I want access to $1 without being hammered with $` and $'.