select returns the currently selected filehandle. So, this is doing what you appear to expect:

select(ERRFP), $| = 1
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then, within its parentheses, it generates a list:

(last_selected_filename, 1)
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Taking the first index of that list ((last_selected_filename, 1)[0]) gives:

last_selected_filename
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Finally, this leaves you with:

select(last_selected_filename);
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In short, you select ERRFP and make it unbuffered, then reselect the original filehandle. All in one line. Haven't seen this is ages. :-)

Update: I just remembered where I did see this last: using format about a decade ago. See perlform - Format Variables for examples of less uglier ways to do the same thing.