Ok, here it is:

Right from the perlop manpage:

Because the transliteration table is built at
compile time, neither the SEARCHLIST nor the
REPLACEMENTLIST are subjected to double quote
interpolation.  That means that if you want to use
variables, you must use an eval():
 
eval "tr/$oldlist/$newlist/";
die $@ if $@;

eval "tr/$oldlist/$newlist/, 1" or die $@;
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Do a perldoc perlop to read all about tr