Thank you. Data::Rmap was exactly what I needed. The example script below converts every value corresponding to a key of 'b' to an array unless it already is an array.

use strict;
use warnings;
use Data::Rmap qw(rmap_hash);
use Data::Dumper;
$Data::Dumper::Sortkeys = 1;
$Data::Dumper::Indent = 1;

my $data = {
  a => {
    b => 1,
    x => 3,
  },
  b => 1,
  c => {
    d => [
      {  b => 3 },
      {  b => [5, 6, 7] },
      {  x => 3},
    ],
  },
};
my $key = 'b';
my @to_fix = rmap_hash { 
    exists($_->{$key}) && (ref($_->{$key}) ne "ARRAY") 
    ? $_ : () 
} $data;
for my $h(@to_fix) {
  $h->{$key} = [ $h->{$key} ];
}
warn Dumper($data);
[download]

Output:

$VAR1 = {
  'a' => {
    'b' => [
      1
    ],
    'x' => 3
  },
  'b' => [
    1
  ],
  'c' => {
    'd' => [
      {
        'b' => [
          3
        ]
      },
      {
        'b' => [
          5,
          6,
          7
        ]
      },
      {
        'x' => 3
      }
    ]
  }
};
[download]