Using a regular expression:

use strict;
use warnings;

my $input_string = 'test';
my $hexchars = '';

foreach my $chars (split(//,$input_string)) {
    $hexchars .= sprintf "%x", ord($chars);
}
print "$input_string\n";
print "$hexchars\n";

my $length = length $hexchars;
my $start = 0;
my $end = $start + $length;

print "length $length\n";
print "start_offset: $start\n";
print "end_offset: $end\n";

for ( $start = 0; $start < $length; $start ++ ) {
    print $start, "\n";
}

while ($hexchars =~ /(.{4})/g) {
    print "$1\n";
}
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