The problem is that \b marks a "word boundary", that is, a place where one side must be a a word character (alphanumeric or _) and the other side isn't a word character. If the text in $symbol begins and ends with word characters, then it does what you want. But if the text in $symbol is, say, a comma, which isn't a word character, then your test is ensuring that it's surrounded on both sides by word characters. So what you want is not to assert a boundary, but rather that $symbol not be preceded by or followed by wordchars.

if ($sentence =~ /(?<!\w)$symbol(?!\w)/){
  # do something
}
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