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use strict;
use warnings;
...
my ( $num, $text ) = unpack( 'A4xA*', $string );

print "[$num][$text]\n";

[1234][this is the remaining string]

use strict;
use warnings;
...
                regex  => sub { $string =~ m[(\d+)\s+(.*)$] },
                split  => sub { split( /(\d+)\s+/, $string, 2 ) },
              } );

            Rate  split  regex unpack
split   789472/s     --   -44%   -70%
regex  1422218/s    80%     --   -45%
unpack 2594984/s   229%    82%     --