use strict;
use warnings;
use Data::Dump qw (pp);

my $accounts = [
    {
           'date' => '1980-03-05',
           'year' => '2019',
           'acc' => '44443',
         },
    {
           'date' => '1999-06-19',
           'year' => '2017',
           'acc' => '54321',
         },
    {
           'date' => '1999-09-19',
           'year' => '2099',
           'acc' => '12345',
         },
    ];
    
my $all = [
         {
           'date' => '2000-01-00',
           'year' => '2003',
           'acc' => '4327',
         },
         
         {
           'date' => '1987-01-03',
           'year' => '2013',
           'acc' => '89997',
         },
         
         {
           'date' => '1980-04-18',
           'year' => '2016',
           'acc' => '239876A',
         }, 
         
         {
           'date' => '1999-06-19',
           'year' => '2017',
           'acc' => '54321',
         }
        ]; 

# add all hashes from $accounts to $all,  but
# exclude account 44443, if it is there

foreach my $href (@$accounts)
{
     push @$all, $href if $href->{acc} ne '44443'; #note string compare
} 

pp $all;
__END__

[
  { acc => 4327, date => "2000-01-00", year => 2003 },
  { acc => 89997, date => "1987-01-03", year => 2013 },
  { acc => "239876A", date => "1980-04-18", year => 2016 },
  { acc => 54321, date => "1999-06-19", year => 2017 },
  { acc => 54321, date => "1999-06-19", year => 2017 },
  { acc => 12345, date => "1999-09-19", year => 2099 },
]

Note: account OP's test $account data did contain a record that already
exists in the $all array. This results in a duplicate record.

See acc=> 54321 above.

There are of course ways to prevent that duplication.
Before suggesting code for that, I'd like to hear from the
OP whether this can really happen in "the real world" and
if such a solution is even needed.