The regular expression // works differently in split then elsewhere:

$ perl -le'
my $x = "1234 abcd 5678";
print $& if $x =~ /[a-z]+/;
print $& if $x =~ //;
print map qq[ "$_"], split /[a-z]+/, $x;
print map qq[ "$_"], split //, $x;
'
abcd
abcd
 "1234 " " 5678"
 "1" "2" "3" "4" " " "a" "b" "c" "d" " " "5" "6" "7" "8"
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Also, the line anchors /^/ and /$/ don't require the /m option to match lines in a string.

The regular expression // works differently in split then elsewhere...

I think I'd consider this just another special-case fixup prior to running split rather than a true difference in the function of m//:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $x = qq{1234 abcd 5678};
 dd split //,      $x;
 dd split /\b|\B/, $x;
"
(1 .. 4, " ", "a" .. "d", " ", 5 .. 8)

(1 .. 4, " ", "a" .. "d", " ", 5 .. 8)
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This is probably just a matter of emphasis and interpretation.

... line anchors /^/ and /$/ don't require the /m option to match lines in a string.

Checking the docs, I recalled seeing this discussed before, but it's another one of those very specialized special cases that evaporates from my memory with time. However, it's not true for the /$/ case (per the docs (or at any rate, the docs say nothing about special-casing it)):

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $x = qq{1234 \n abcd \n 5678};
 dd split /^/, $x;
 dd split /$/, $x;
"
("1234 \n", " abcd \n", " 5678")

"1234 \n abcd \n 5678"
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Give a man a fish:  <%-{-{-{-<

The regular expression // works differently in split then elsewhere

I think it is actually the other way around — in most contexts, m// is special (it refers to the most recent pattern without duplicating that pattern), while in split, // is literally the empty regex, which matches the zero-length empty string.