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use strict;
use warnings;
...
   concat => sub { my $ret; $ret .= $_ for @arr; return $ret; },
   join   => sub { my $ret = join q{}, @arr; return $ret; },
   } );

$ ./spw1003500
         Rate concat   join
concat  994/s     --   -70%
join   3310/s   233%     --
$