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use strict;
use warnings;
...
           unpack => sub { @sub_fields = unpack '(A4)*', $string },
           substr => sub { @sub_fields = map { substr $string, 5*$_, 5
+ } 0..length( $string )/5-1 },
          });

             Rate substr unpack regex1 regex2
substr   696486/s     --   -57%   -94%   -94%
unpack  1603093/s   130%     --   -85%   -86%
regex1 10731041/s  1441%   569%     --    -4%
regex2 11165392/s  1503%   596%     4%     --