%xyz is a hash. In it, under the key stored in $item[1], lives a values which is an anonymous hash ( { } ). In that anonymous hash, under the key is, an anonymous array ([ ]) is stored, which is dererferenced with the construct @{ }. See perlref.

To that anonymous array, the content of another anonymous array is pushed, which is stored at slot 4 of the array @item.

Or, said as comments to the code,

#!/usr/bin/perl

use Data::Dumper;
$Data::Dumper::Indent = 1;

my (@item, %xyz);
$item[1] = 'some key';
$item[3] = [ qw(foo bar quux) ];

push            # push to an array - which is
  @{            # a dereferenced anonymous array which lives
    $xyz{       # in the hash %xyz, under the key stored in
      $item[1]  # element 2 of the array @item
    }           # inside that keys value which is an
    {is}        # anonymous hash, there under the key 'is'
  },            # - push to that array -
  @{            # the contents of an anonymous array
    $item[3]    # stored as element 4 in the array @item
  };

print Data::Dumper->Dump([\%xyz] , [qw( xyz )]);
print Data::Dumper->Dump([\@item], [qw( item )]);
__END__
$xyz = {
  'some key' => {
    'is' => [
      'foo',
      'bar',
      'quux'
    ]
  }
};
$item = [
  undef,
  'some key',
  undef,
  [
    'foo',
    'bar',
    'quux'
  ]
];
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