Q: How would you do it manually?
A: You would recall which you have already seen, and only output if it were not in that list.

Q: How to do that in code?
A: One way would be to put the name into a hash, and only output if it were not present. In my example code below (which I used data from an array instead of a file, but the logic within the while loop is the same as if processing a file), I split the line into two (2) parts based on m/\s+/ (one or more whitespace characters, which could be spaces, tabs, etc). I then check if the name exists as a key in the hash (%seen); if not, I output the line. After the check, I increment the value of the hash element with the name as the key.

Output:

$ ./11148698-00.pl
id  name
123 john
11  peter
87  helen
[download]

Code:

#!/usr/bin/env perl

use strict;
use warnings;
use utf8;

my @data = (
    q/id  name/,
    q/123 john/,
    q/34  john/,
    q/567 john/,
    q/11  peter/,
    q/899 peter/,
    q/87  helen/,
);

my %seen;
while ( my $str = shift @data ) {
    chomp $str;
    my @part = split /\s+/, $str, 2;
    if ( not exists $seen{ $part[1] } ) {
        print $str, qq{\n};
    }
    $seen{ $part[1] }++;
}
print qq{\n};
[download]

Hope that helps.