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use warnings;
use strict;
use Benchmark qw/:all/;
...

my @words = $code =~ /_[\w_]* | [a-zA-Z]\w*_\w*/gx;
print "\nWords: ", join(', ', @words);

            Rate split  grep regex
split 10581141/s    --  -22%  -49%
grep  13630260/s   29%    --  -34%
regex 20760455/s   96%   52%    --

Words: is_, _be, some_sample, __code, __0K__, _, _, _