This should do what you want:

#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';

my $word  = 'helen';
my $input = 'xxxhexxxxxxxx xle xxnxle nxxx xxnxxx xnxxhexx nxxxxx xlex
+xxxxx nxnx xxxx';

my $regex = '([^ \b]*' . join('[^ \b]*).*?([^ \b]*', $word =~ /(..?)/g
+) . '[^ \b]*)';

my @matches = $input =~ /$regex/g;
say for @matches;
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The regex is constructed from the $word dynamically, so you do not have to know the length of it in advance.