I　google it, but nothing useful.

[ddg://"$^E" site:perl.org] -> "$^E" site:perl.org

[google://"$^E" site:perl.org] -> "$^E" site:perl.org

[google://"$^E" site:cpan.org] -> "$^E" site:cpan.org

[perldoc://"$^E"] -> "$^E"

Yeah, google seems to have gotten crappier, meh

perldoc -v "$^E"
    $EXTENDED_OS_ERROR
    $^E     Error information specific to the current operating system
+. At
...
[download]

$ perldoc perltoc |grep "\^E"
            , $COMPILING, $^C , $DEBUGGING, $^D , ${^ENCODING} , %ENV 
+,
            ${^CHILD_ERROR_NATIVE} , $EXTENDED_OS_ERROR, $^E ,
        $^E is meaningful on Win32
            $^E, $^H, $^M
        %ENV, CRTL_ENV, CLISYM_[LOCAL], Any other string, $!, $^E, $?,
+ $|
[download]

[doc://perlvar#$^E] -> $^E

You cant go wrong with a classic :) Tutorials: How to RTFM