OK, I'm trying things. Here's what I've got.

This first one is the nominal - it works and accomplishes what I want. I've decluttered from above so I'm only dealing with one hash and not using a temporary variable for the big, modular math. I've added a nice map statement to turn %p2C inside out, becoming %C2p. 

sub key {
    $kp= shift;
    $caesar= shift;
#version 1 - this works, the starting point
    foreach (split(//, $kp),'a'..'z') {
        if (not exists $p2C{$_}) {
            $p2C{$_}=chr(ord('A')+(($a++)+$caesar)%26);
        }
    }

    %C2p = map {$p2C{$_} => $_} (keys %p2C);
    $p2C{' '}=' ';
    $C2p{' '}=' ';
}    

sub printKeys {
    my $href = shift;
    print "\nCT: ";
    print(($_).' ') foreach (sort keys %{$href});
    print "\npt: ";
    print(($$href{$_}).' ') foreach (sort keys %{$href});
    print "\n";
}

key ('thequickbrownfoxjumpsoverthelazydogs',10);

printKeys(\%p2C);
printKeys(\%C2p);

[download]

This produces output thus: 

C:\chas_sandbox>foreachasmap.pl

CT:   a b c d e f g h i j k l m n o p q r s t u v w x y z
pt:   F S Q I M X J L P Z R E A W U B N T C K O D V Y H G

CT:   A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
pt:   m p s v l a z y d g t h e q u i c k b r o w n f x j

C:\chas_sandbox>

[download]

Next, I've turned the if statement into a ( ? : ) operator. In version 1 I was looking for if (not exists . . .) so I needed something to do if it did exists and assigned to a hash key that I knew I wouldn't use, '#'. I've spotted my bug here - the autoincrement of $a happens every time now instead of only if (not exists . . .). Any suggestions for that or am I a victim of my own ambition? 

sub key {
    $kp= shift;
    $caesar= shift;
#version 1 - this works, the starting point
#     foreach (split(//, $kp),'a'..'z') {
#         if (not exists $p2C{$_}) {
#             $p2C{$_}=chr(ord('A')+(($a++)+$caesar)%26);
#         }
#     }

#version 2 - this doesn't work - because autoincrement $a++ gets off w
+hen the key already exists!
    foreach  (split(//, $kp),'a'..'z') {
        $p2C{( exists $p2C{$_} ? '#' : $_)} = chr(ord('A')+(($a++)+$ca
+esar)%26);
    }

    %C2p = map {$p2C{$_} => $_} (keys %p2C);
    $p2C{' '}=' ';
    $C2p{' '}=' ';
}    

sub printKeys {
    my $href = shift;
    print "\nCT: ";
    print(($_).' ') foreach (sort keys %{$href});
    print "\npt: ";
    print(($$href{$_}).' ') foreach (sort keys %{$href});
    print "\n";
}

key ('thequickbrownfoxjumpsoverthelazydogs',10);

printKeys(\%p2C);
printKeys(\%C2p);

[download]

Here's the second output. 

C:\chas_sandbox>foreachasmap.pl

CT:   # a b c d e f g h i j k l m n o p q r s t u v w x y z
pt:   T N S Q Q M X S L P A R M C W U D N T E K O G V Z P O

CT:   A C D E G K L M N O P Q R S T U V W X Z
pt:   j m p s v t h l q z i c k b # o w n f x

[download]

The last version here would be what I'm really after. I'm just moving things around according to the doc for map: 

    %hash = map { getkey($_) => $_ } @array;

[download]
is just a funny way to write 
    %hash = ();
    foreach $_ (@array) {
    $hash{getkey($_)} = $_;
    }

[download]

I have a foreach, a list and I'm mapping a key to a value in a hash, so it seems it should be no more complicated than this. Here's the code: 

sub key {
    $kp= shift;
    $caesar= shift;
#version 2 - this doesn't work - because autoincrement $a++ gets off w
+hen the key already exists!
#     foreach  (split(//, $kp),'a'..'z') {
#         $p2C{( exists $p2C{$_} ? '#' : $_)} = chr(ord('A')+(($a++)+$
+caesar)%26);
#     }

#version 3 - this doesn't work - why?
    %p2C = map {( exists $p2C{$_} ? '#' : $_ )=> chr(ord('A')+(($a++)+
+$caesar)%26) } (split(//, $kp),'a'..'z');
    %C2p = map {$p2C{$_} => $_} (keys %p2C);
    $p2C{' '}=' ';
    $C2p{' '}=' ';
}    

sub printKeys {
    my $href = shift;
    print "\nCT: ";
    print(($_).' ') foreach (sort keys %{$href});
    print "\npt: ";
    print(($$href{$_}).' ') foreach (sort keys %{$href});
    print "\n";
}

key ('thequickbrownfoxjumpsoverthelazydogs',10);

printKeys(\%p2C);
printKeys(\%C2p);

[download]

I don't even get the same output as in version 2: 

C:\chas_sandbox>foreachasmap.pl

CT:   a b c d e f g h i j k l m n o p q r s t u v w x y z
pt:   U V W X Y Z A B C D E F G H I J K L M N O P Q R S T

CT:   A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
pt:   g h i j k l m n o p q r s t u v w x y z a b c d e f

C:\chas_sandbox>

[download]

I know this may all seem academic since it's working in version 1, but, well, it's academic and I'd like to learn something.

I humbly seek wisdom.

In reply to Re^2: adapting foreach to map (why?) by goibhniu
in thread adapting foreach to map by goibhniu

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