use Benchmark;

$img = "http://www.my-website.be/gif/wwwtitle.gif";
timethese(1000000, {
    'regexp1' => sub { my($file) = $img =~ m#.*/(.*)# },
    'regexp2' => sub { $img =~ m#[^/]+$#; my $file=$& },
    'regexp3' => sub { $img =~ m#.*/#; my $file=$' },
    'replace' => sub { $file=$img; $file =~ s#^.*/## },
    'split'   => sub { my($file) = (split /\//, $img)[-1] },
    'substr'  => sub { my $file  = substr $img, rindex($img, '/') + 1 }
});