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use Benchmark;

...
    'split'   => sub { my($file) = (split /\//, $img)[-1] },
    'substr'  => sub { my $file  = substr $img, rindex($img, '/') + 1 
+}
});

Benchmark: timing 1000000 iterations of regexp1, regexp2, regexp3, rep
+lace, split, substr...
   regexp1: 26 wallclock secs (20.06 usr +  0.06 sys = 20.12 CPU)
...
   replace: 17 wallclock secs (13.05 usr +  0.03 sys = 13.08 CPU)
     split: 30 wallclock secs (24.56 usr +  0.09 sys = 24.65 CPU)
    substr: 11 wallclock secs ( 8.46 usr +  0.02 sys =  8.48 CPU)