I need to count the number of occurences of every possible 1, 2, and 3 letter combination in this sequence.

I'd use substr() and build up a hash as I made one pass through the string.

how I can use a variable in a tr// regexp?

You must use eval.

Update: Code for your first question:

my $length = length $string;
my %seen;
for my $i (0 .. $length - 1) {
  $seen{ substr($string, $i, 1) } ++;
  $seen{ substr($string, $i, 2) } ++ if $i < $length - 1;
  $seen{ substr($string, $i, 3) } ++ if $i < $length - 2;
}
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-sauoq
"My two cents aren't worth a dime.";