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

#!/usr/bin/perl -w

...
    "substr" => sub { ($char) = substr($string,4,1);},
    "silly"  => sub { local $string; $char = chop $string while length
+ $string > 4;},
    });

Benchmark: timing 150000 iterations of match, silly, substr, unpack...
     match:  4 wallclock secs ( 2.78 usr +  0.01 sys =  2.79 CPU) @ 53
+763.44/s (n=150000)
...
            (warning: too few iterations for a reliable count)
    substr:  0 wallclock secs ( 0.48 usr +  0.00 sys =  0.48 CPU) @ 31
+2500.00/s (n=150000)
    unpack:  2 wallclock secs ( 1.41 usr +  0.11 sys =  1.52 CPU) @ 98
+684.21/s (n=150000)