My guess is that the OP wants to edit the values of an existing hash. Each value that is not unique should be made so by appending an integer.

use strict;
use warnings;
use Data::Dumper;
my %example = (
    a => 'red',
    b => 'blue',
    c => 'red',
    d => 'green',
    e => 'red',
);

my %desired = (
    a => 'red',
    b => 'blue',
    c => 'red1',
    d => 'green',
    e => 'red2',
);

       
my %seen;
while (my ($letter, $color) = each %example) {
    if ($seen{$color}++) {
        $example{$letter} = $color . $seen{$color};
        next;
    }
    $example{$letter} = $color
}
print Dumper( \%desired, \%example );
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UPDATE: Added example.