Thanks for the heads up. It also seems the string "-1" in the second file shortens what is worked on there and leaves the rest as plaintext.

This would be fixed by using undef$/; rather than $/--;. However, this is supposed to be using the record size setting mentioned in perlvar rather than stringification. According to perlvar, for 5.22 and 5.24:

Trying to set the record size to zero or less is deprecated and will cause $/ to have the value of "undef", which will cause reading in the (rest of the) whole file.

... and using $/-- does leave the variable with a numeric value of -1. In a suitably old version of perlvar (that for 5.10.1), the text reads:

Trying to set the record size to zero or less will cause reading in the (rest of the) whole file.

... and in perl 5.10.1 we also get that the numeric value has been successfully set to -1.

perl -e '$/--; printf "%d\n", $/; printf "%f\n", $/; $foo = <>; $bar =
+ <>; $_ = $foo ^ $bar;  print $_ . "\n"' in1 in2
-1
-1.000000
'1
[download]

where in1 is:

abc-1def
[download]

and in2 is:

12345678
[download]

while the proper output for the important line is:

PPPRR^
[download]

This appears to my eyes to be a bug in perl or the documentation, but in the bigger picture is what happens when one pushes into dark corners in the name of cleverness.