Use List::Util::reduce. You have a serious problem if you use the approach you selected. If you have already added "a", "b" = 1, what happens when you say "a", "b", "c" = 2? Using your existing code you will immediately die on symbolic references. Better, solve the problem by putting the child node references into a separate key and store your values in another.

use List::Util 'reduce';
use Carp 'carp';

# a.tree => b.tree => c.tree => ... z.value

sub put_element {
    'HASH' eq ref( my $root = shift() )
        or carp "put_element( \ %..., [ KEY ], $val ) must be a hash r
+eference";
    'ARRAY' eq ref( my $keys = shift() )
        or carp "put_element( \ %..., [ KEY ], $val ) must be given an
+ array reference";
    my $value = shift;

    my $leaf = reduce { $a->{$b}{'tree'} ||= {} } $root, @{$keys}[ 0 .
+. $#$keys - 1 ];
    $leaf->{$keys->[-1]}{'value'} = $value;
}
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