If I read the requirement of AM correct, he wants to check for the presence of a $symbol in a $sentence, where the beginning and end of $symbol should also be treated as word boundaries, provided they are word characters.

For this I would propose the following regex (using conditional pattern as suggested by japhy above):

if ($sentence =~ /(?(?=\w)(?:\b))$symbol(?(?<=\w)(?:\b))/) {
  # do something
}
[download]