Ah - got it (I think)

My actual script reads the CSV via CSV_XS, with 'binary' on, so I'm guessing that's what alerts Perl.

Many thanks.

UPDATE!

Okay, so I altered my test-script to explicitly open utf-8, and now the test-script throws an error at both locations, although my main script continues to only throw an error locally - go figure.

In any event, opening my output file with ':encoding(UTF-8)' has fixed the issue.

map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2
-$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
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