my %people = (
  1 => { first_name => 'Bob',     last_name => 'Smith',   topic => 'Anthropology' },
  2 => { first_name => 'Sandy',   last_name => 'Smith',   topic => 'Sociology'    },
  3 => { first_name => 'Tom',     last_name => 'Sanders', topic => 'English'      },
  4 => { first_name => 'Tina',    last_name => 'Lacy',    topic => 'Social Work'  },
  5 => { first_name => 'Anthony', last_name => 'Sanders', topic => 'English'      },
);

##</code><code>##

#!/usr/bin/perl;

use strict;
use warnings;
use feature qw/ say /;

my %people = (
  1 => { first_name => 'Bob',     last_name => 'Smith',   topic => 'Anthropology' },
  2 => { first_name => 'Sandy',   last_name => 'Smith',   topic => 'Sociology'    },
  3 => { first_name => 'Tom',     last_name => 'Sanders', topic => 'English'      },
  4 => { first_name => 'Tina',    last_name => 'Lacy',    topic => 'Social Work'  },
  5 => { first_name => 'Anthony', last_name => 'Sanders', topic => 'English'      },
);

my @last_names = map { $_->{'last_name'} } values %people;

say for sort @last_names;

__END__

##</code><code>##

$ perl 1142844.pl
Lacy
Sanders
Sanders
Smith
Smith