The following code takes 250x longer on my machine when I uncomment any one line in the 'uncomment_one' subroutine. I am running 5.8.8 on a 64-bit 2.6.9 linux. A test on a different machine with a newer kernel (same perl version) showed a 50x degradation, so did testing with 5.10.1. I have not tried on a newer version. 
use strict;
use warnings;
use Time::HiRes qw(time);

sub uncomment_one {
    #my $test = $`;
    #my $test = $';
    #my $test = $&;
}

my $in = "abcdefghijklmnopqrstuvwxyz\n" x 20_000;
my @a;    
my $r = '.*';
my $start = time;
while ( $in =~ /^($r)$/mg){
    push @a, $1;
}
print "Took " . (time - $start) . " seconds";

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I am aware that perlvar says the following about these three special variables:
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches
Since it is possible to rewrite the above in a way that doesn't suffer from that large of a slowdown I wonder if I have hit something worthy of a bug report though? Or is this expected behavior? (Splitting the input into multiple lines and doing a match on each line, rather than using /mg is much much faster in this instance)

Update: Astute readers will notice that the special characters are being used in a sub that's never called. This isn't a typo. The code above is fully functional and demonstrate the effect I am talking about.

In reply to Impact of special variables on regex match performance by roubi

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