$ cat testLocal.pl
#!/usr/bin/perl -w

use strict;

use Benchmark qw(cmpthese);

my @a=qw(a x z);
# if you increase the number of test strings, the differences
# get even more pronounced...
my @strings=qw(a x z vvvvv vvvvva vvvvvb vvvvvx vvvvvz aaa axz azz axx);

sub localWhile{
    local $"="";
    my $count;
    foreach(@strings) {
        $count++ if /^[@a]{3}$/;
    }

    die unless $count==4;
}

sub whileLocal{
    my $count;
    foreach(@strings) {
        local $"="";
        $count++ if /^[@a]{3}$/;
    }

    die unless $count==4;
}

sub localWhileQR{
    my $count;
    local $"="";
    my $re=qr/^[@a]{3}$/;
    foreach(@strings) {
        $count++ if /$re/;
    }

    die unless $count==4;
}

cmpthese(-3,
        {
            localWhile => \&localWhile,
            localWhileQR => \&localWhileQR,
            whileLocal => \&whileLocal
        }
    );