I was replying to jettero who said "substr doesn't actually modify the string like splice modifies arrays" and there was no mention of hash keys in his post.

I know. I just related your answer to the topic. Out of context answers are dangerous.

And actually, hash keys are strings

I never said they weren't. I said they weren't Perl strings. They're not C strings either. They're C strings with a flag byte appended.

struct hek {
    U32         hek_hash;       /* hash of key */
    I32         hek_len;        /* length of hash key */
    char        hek_key[1];     /* variable-length hash key */
    /* the hash-key is \0-terminated */
    /* after the \0 there is a byte for flags, such as whether the key
       is UTF-8 */
};

typedef struct hek HEK;
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hash keys are strings, just not modifiable strings, but there is no error if you try to modify them:

Not so. What you call a hash key is modifiable.

use warnings;
use strict;
my %x = qw/abcd efgh ijkl mnop/;
for (%x) {
   substr $_, 0, 2, "";
   print("$_\n");
}
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kl
op
cd
gh
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That's because it's a copy of the hash key, not the hash key itself.