Or declare @array within the while loop. This avoids making a copy of the array to make the anonymous reference.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 1;

my %hash;
while (my $rec = <DATA>){
  chomp $rec;
  # declare the array
  my ($key, @array) = split(/ /, $rec, 2);
  $hash{$key} = \@array;
}
print Dumper \%hash;

__DATA__
1 2 3 4
5 6 7 8
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$VAR1 = {
  '1' => [
    '2 3 4'
  ],
  '5' => [
    '6 7 8'
  ]
};
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