Challenge 024 task 2

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diff --git a/challenge-024/jo-37/perl/ch-2.pl b/challenge-024/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl
+
+use v5.16;
+use warnings;
+use Data::Dump;
+use experimental 'postderef';
+
+### Input and Output
+
+dd build_index(shift);
+
+
+### Implementation
+
+sub build_index {
+    my %index;
+    # Match words in the input string and record all starting positions.
+    () = shift =~ /(\w+)(?{push $index{$^N}->@*, pos() - length($^N)})/g;
+    \%index;
+}