Hmm, thinking more about your problem, assuming you really need your array of hashes for some other reasons than what you have shown us, and also assuming that your labels are unique, you could build an auxiliary hash of hashes providing direct access to the values that you want to update, through the magics of references, i.e. store references to the data items that you need to change in an additional hash.

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

# Defaults
my @t = (
   {
      'label' => 'kept',
      'value' => '0',
   },
   {
      'label' => 'notkept',
      'value' => '0',
   },
   {
      'label' => 'repaired',
      'value' => '0',
   },
);

# Create another structure (hash) to access the same data in a faster 
+way
my %faster_access;
for my $valref (@t) {
      $faster_access{$valref->{'label'}} = $valref;
}

# Get this from another sub
my $a = [
   [ 'kept', '1', ],
   [ 'repaired', '3' ]
];

# Overwrite defaults 

for my $i ( @{ $a } ) {
     $faster_access{$i->[0]}{value} = $i->[1];
}

warn Dumper( \$a );
warn Dumper( \@t );
[download]

The auxiliary HoH makes it possible to access directly (and much faster) to the data that you want to update. This is the output of the program:

$ perl defaults.pl
$ perl  defaults.pl
$VAR1 = \[
            [
              'kept',
              '1'
            ],
            [
              'repaired',
              '3'
            ]
          ];
$VAR1 = [
          {
            'value' => '1',
            'label' => 'kept'
          },
          {
            'value' => '0',
            'label' => 'notkept'
          },
          {
            'value' => '3',
            'label' => 'repaired'
          }
        ];
[download]

As you can see, the @t array has been modified indirectly thanks to the changes made to the %faster_access containing references to the values that you need to update. There is no longer a need to browse through the whole AoH when you want to update it, which is handy if you need to update many times in the course of your program execution.