$char = chop $string while length $string > 4;

##</code><code>##

#!/usr/bin/perl -w

use Benchmark;

$string = "hello world I am a string of some sort of length";

timethese(150_000, {
    "match" => sub { ($char) = $string =~ /^.{4}(.)/;},
    "unpack" => sub { ($char) = unpack("x4 A1", $string);},
    "substr" => sub { ($char) = substr($string,4,1);},
    "silly"  => sub { local $string; $char = chop $string while length $string > 4;},
    });

##</code><code>##

Benchmark: timing 150000 iterations of match, silly, substr, unpack...
     match:  4 wallclock secs ( 2.78 usr +  0.01 sys =  2.79 CPU) @ 53763.44/s (n=150000)
     silly: -1 wallclock secs ( 0.26 usr + -0.10 sys =  0.16 CPU) @ 937500.00/s (n=150000)
            (warning: too few iterations for a reliable count)
    substr:  0 wallclock secs ( 0.48 usr +  0.00 sys =  0.48 CPU) @ 312500.00/s (n=150000)
    unpack:  2 wallclock secs ( 1.41 usr +  0.11 sys =  1.52 CPU) @ 98684.21/s (n=150000)