use strict; use warnings; 
my $data=[]; 
$data->[0]={code=>'YYK',expdate=>'fixme'}; 
$data->[1]={code=>'YYK',expdate=>'fixme'}; 
$data->[2]={code=>'YYK',expdate=>'fixme'}; 
$data->[3]={code=>'YYK',expdate=>'fixme'}; 
$data->[4]={code=>'YYK',expdate=>'fixme'}; 
$data->[5]={code=>'XXY',expdate=>'fixer'}; 
$data->[6]={code=>'YYK',expdate=>'fixme'}; 
$data->[7]={code=>'YYK',expdate=>'fixme'}; 

my ($xxy, $yyk);
for (@$data) {
  $xxy = $_ if $_->{code} eq 'XXY';
  $yyk = $_ if $_->{code} eq 'YYK';
  if ($xxy && $yyk) {
    $yyk->{expdate} = $xxy->{expdate};
    last;
  }
}

use Data::Dumper; 
print Dumper($data);
[download]

$VAR1 = [
          {
            'expdate' => 'fixme',
            'code' => 'YYK'
          },
          {
            'code' => 'YYK',
            'expdate' => 'fixme'
          },
          {
            'expdate' => 'fixme',
            'code' => 'YYK'
          },
          {
            'expdate' => 'fixme',
            'code' => 'YYK'
          },
          {
            'expdate' => 'fixer',
            'code' => 'YYK'
          },
          {
            'code' => 'XXY',
            'expdate' => 'fixer'
          },
          {
            'code' => 'YYK',
            'expdate' => 'fixme'
          },
          {
            'expdate' => 'fixme',
            'code' => 'YYK'
          }
        ];
[download]

Wasnt the first one it found in index 0?