Here's one way to get the output you want, given your input...

use warnings;
use strict;

my %names;
while (<DATA>) {
    next if /^ID/;
    my ($first, $last) = (split)[1 .. 2];
    $last =~ s/,//;
    push @{ $names{$last} }, $first;
}
print "$_ Rhodes\n" for @{ $names{Rhodes} };

__DATA__
ID     NAME   FAMILYTAG   EFFECT
001  John      Black               Positive
002  Kate      Rhodes, Mich           Positive
003  Aaron   Sunders          Negative
004  Shirley  Rhodes          Negative
005  Dexter    Sunders        Positive
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See also:

• perldsc