use strict;
use warnings;
use Data::Dumper;

my @columns = qw/ title1 title2 title3 title4 
                  field1 field2 field3 field4 /;

my $struct_ref = [ @columns[4..7] ]; # place the 'fields' into an array ref.

$#columns = 3; # truncate the original CSV row array, @columns.

foreach my $col ( reverse @columns ) {
    $struct_ref = { $col => $struct_ref };
}

print Dumper $struct_ref;