$ iconv -f UTF-8 -t us-ascii 1.pl -o 1.us-ascii.pl
iconv: illegal input sequence at position 72
$
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I finally realized what that was when I took a look inside the unexpected partial file that resulted from this command: 1.us-ascii.pl:

$ cat 1.ascii.pl 
#!/usr/bin/perl -w
use 5.011;
#use utf8;
use Devel::Peek;
my $string = 'G$
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So, position 72 was where the first umlaut occurred, and now I at least understand the error.