Do you have use warnings; at the top of your program? It appears not, as this code:

[nick:~/monks]$ perl -Mstrict -Mwarnings -e"
my @foo; push @foo = {bar=>'baz'};
"
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produces this output for me:

Useless use of push with no values at -e line 2.
push on reference is experimental at -e line 2.
Not an ARRAY reference at -e line 2.
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As you can see the first line of the output is more descriptive about the error, which often helps you find it more quickly. So always use warnings;.

As an aside: You don't show what you do with your array, but I am wondering if it might not be more useful for you to have your invoices stored in a hash so you can retrieve them by index:

#! perl
use strict;
use warnings;

my $cast = list_characters()->{'cast'};

print "The Human is $cast->{'Human'}->{'fname'} $cast->{'Human'}->{'ln
+ame'}.";
print " He is always displaying $cast->{'Human'}->{'characteristic'}.\
+n";

sub list_characters {
  my $Guide;

  $Guide->{'cast'}->{'Human'} = { 
    fname          => 'Arthur',
    lname          => 'Dent',
    characteristic => 'bewilderment',
  };  

  $Guide->{'cast'}->{'Betelgeusian'} = { 
    fname          => 'Zaphod',
    lname          => 'Beeblebrox',
    characteristic => 'narcissism',
  };  

  $Guide->{'cast'}->{'Robot'} = { 
    fname          => 'Marvin',
    lname          => 'the Paranoid Android',
    characteristic => 'depression',
  };  

  return $Guide;
}

__END__
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Output:

[09:07][nick:~/monks]$ perl 1139851.pl
The Human is Arthur Dent. He is always displaying bewilderment.
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