enough theory, now practical code: 

use strict;
use warnings;
use Data::Dump qw/pp/;
use Scalar::Util qw/reftype/;

sub walk {
  my ($entry,$code) =@_;
  my $type = reftype($entry);
  $type //= "SCALAR";

  if    ($type eq "HASH") {
    walk($_,$code) for values %$entry;
  }
  elsif ($type eq "ARRAY") {
    walk($_,$code) for @$entry;
  }
  elsif ($type eq "SCALAR" ) {
    $code->($_[0]);        # alias of entry
  }
  else {
    warn "unknown type $type";
  }
}


my $test = { a => [2, 3, [4, { b => 42 }]], c => 5 };

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

[download]

Output 

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

[download]

Cheers Rolf

( addicted to the Perl Programming Language)

UPDATE

of course you could extend walk to call optional $coderefs on every node and specify them as named parameters:

 walk $ref , SCALAR => sub { print $_[0] } , ARRAY => sub { Dump $_[0] }, ...

But I think the original code is so small thats its easier to copy and manipulate it in place for different needs.

In reply to Re^2: Manipulate deepest values in complex hash structures by LanX
in thread Manipulate deepest values in complex hash structures by RecursionBane

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