use unshift to add items to the start of an array.

If by "null" you mean undef, then try this:

use Data::Dumper;

my @array1 = qw( 1 2 3 );
my @array2 = qw( 4 );

unshift @array2, undef for @array1;
print Dumper( \@array2 );
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output:

$VAR1 = [
          undef,
          undef,
          undef,
          '4'
        ];
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