Perl - Splitting a string
- Use the strict and warnings pragmas

This helps catch many errors at an early stage.

- Declare lexical variables with my instead of using package variables

If you define variables without having declared them they will be defined as package variables (which are seen by all code in your package). Note that if you use the strict pragma you need to declare package variables with our.

- use say instead of print

Since perl version 5.10 you can use say to print a line and add the line terminator (newline character) automatically. Just remember to enable the the feature with use feature qw(say).

- Unpack arguments to a function/method from the @_ array for clarity

Prefer my ($str, $delim) = @_ over my $str = $_[0]; my $delim = $_[1]

- Use $array[$N] to refer to the ($N+1)th element of @array.

In you code you used @_[1] to refer to the second element of the @_ array. The correct syntax is to use $_[1].

- Do not use parenthesis around argument for builtin functions if not necessary.

In Perl parenthesis around function arguments is optional. A common style is to avoid parenthesis around builtin function calls. This reduces visual clutter and disambiguates built-in functions from user functions, see also What is the reason to use parenthesis-less subroutine calls in Perl?

- Don't declare empty arrays with empty parenthesis. Simply use my @arr;

- Return a reference to an array and not an array value.

By returning a reference you avoid copying, but see also In perl, when assigning a subroutine's return value to a variable, is the data duplicated in memory?

- Don't reinvent the wheel, use the Perl builtin function split

You tagged your question with [reinventing-the-wheel] so I assume this is for learning purposes only. 

Here is a revised version of your code that implements the above comments:

use feature qw(say);
use strict;
use warnings;

{ # &lt;-- create a scope so lexical variable does not "leak" into the subs below

    my $test = split_string("This is a test to ensure this works correctly.");
    foreach my $element (@$test) {
        say $element;
    }
}

sub split_string {
    my ( $string, $delimiter ) = @_;

    $delimiter //= " ";
    my @result;
    my $temp = "";

    for my $i (0..(length $string)) {
        my $char = substr $string, $i, 1;
        if (($char eq $delimiter) or $i == (length $string)) {
            push @result, $temp;
            $temp = "";
        } else {
            $temp .= $char;
        }
    }
    return \@result;
}