You are also, in what I assume is a typo, referencing two different hashes: $hash_ref and $interface_hash_ref.

Per kennethk's reply, what you have might be more conscisely written (assuming use warnings; and use strict;) as:

my @interfaceref_array;

for my $name (@list_of_junk) {

    my $hash_ref = {  # curly brackets enclose
        junk_name => $name,  # quotes not needed
        two       => result_of_function($name),
        };

    do_some_stuff();

    $hash_ref->{three} = "toilet";  # quotes needed for value

    push @interfaceref_array, $hash_ref;

    }
[download]

Or even as:

my @interfaceref_array;

for my $name (@list_of_junk) {

    my %hash = (  # parentheses enclose
        junk_name => $name,
        two       => result_of_function($name),
        );

    do_some_stuff();

    $hash{three} = "toilet";

    push @interfaceref_array, \%hash;

    }
[download]