(my $p = shift) =~ tr/-\0-\377/\0\377/;

In the  tr/-\0-\377/\0\377/ expression, the '-' (hyphen) character appears twice in the search list: initially, and also within the  \0-\377 range. In tests I did with some Win32 Perls in the range 5.8 to 5.14, the test code

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'XXXooX';
 (my $t = $s) =~ tr/XoX/ab/;
 print qq{'$t'};
"
'aaabba'
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(and identically for  tr/X\x00-\xff/ab/) always produced the same result: the leftmost occurrence of a character in the search list is selected for matching to and replacement by the corresponding character in the replacement list.

I considered using the much neater  tr/-\0-\377/\0\377/ version, but I couldn't find anything in the docs to guarantee the behavior shown in my tests must always prevail. Despite the tests, I didn't feel comfortable using an "undocumented feature". Do you know of any documentation of this "leftmost match" feature in the  tr/// built-in? In a regex, the rule would be "leftmost longest match", but  tr/// isn't really a regex, it's a transliterator — isn't it?