Anonymous Monk,
Since you were not overly verbose on your requirements, this is the best I can offer:

#!/usr/bin/perl
use strict;
use warnings;

my $prev_col;
my $counter;
while ( <DATA> ) {
    chomp;
    my ($col, $rest) = $_ =~ /^([^\s]+)(\s.*)$/;
    if ( ! defined $prev_col || $col ne $prev_col ) {
        $counter++;
        $prev_col = $col;
    }
    print "$col $counter$rest\n";
}
__DATA__
aa one two three
aa four five six
xy z
xy a
xy b
cc asdfasdf
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