However, there are other problems with what you're doing.

You're repeating that interpolation on every trip through the loop, when you could be using qr// to compile your regex once and then forget about it.

Here are some comparisons of a few of the other solutions mentioned in this thread, along with one where the regex is pre-built with qr//. These results are with 5.6.1 on cygwin; YMMV.

$ perl testLocal.pl
Benchmark: running localWhile, localWhileQR, whileLocal, each for at l
+east 3 CPU seconds...
localWhile:  4 wallclock secs ( 3.02 usr +  0.00 sys =  0.02 CPU) @ 28
+894.20/s (n=87116)
localWhileQR:  4 wallclock secs ( 3.17 usr +  0.01 sys =  3.18 CPU) @ 
+45765.70/s (n=145718)
whileLocal:  4 wallclock secs ( 3.09 usr +  0.00 sys =  3.09 CPU) @ 20
+884.62/s (n=64617)

                Rate   whileLocal   localWhile localWhileQR
whileLocal   20885/s           --         -28%         -54%
localWhile   28894/s          38%           --         -37%
localWhileQR 45766/s         119%          58%           --
[download]

$ 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
        }
    );
[download]