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use strict;
use warnings;
...
pp $test;
walk $test, sub { $_[0]+=100 };
pp $test;

/usr/bin/perl -w /tmp/walker.pl 
{ a => [2, 3, [4, { b => 42 }]], c => 5 }
{ a => [102, 103, [104, { b => 142 }]], c => 105 }