use warnings;
use strict;

my %test = (11 => "reagen", 26 => "daniel", 13 => "joe");

delete $test{13};
print "$_ :: $test{$_}\n" for sort keys %test;

##</code><code>##

11 :: reagen
26 :: daniel

##</code><code>##

use warnings;
use strict;

my %test = (11 => "reagen", 26 => "daniel", 13 => "joe");

for (sort keys %test) {
    delete $test{$_ - 13} if exists $test{$_ - 13};
}

print "$_ :: $test{$_}\n" for sort keys %test;

##</code><code>##

11 :: reagen
26 :: daniel