$currentSentence =~ s/[\ba\b|\ban\b|\bthe\b]//g;

The main problem with the character class  [\ba\b|\ban\b|\bthe\b] in the quoted substitution is that it's a character class. The  \b resolves, I think, to a backspace control character (or maybe just a plain old  'b' character) and not a word boundary assertion as seems to be the intention. Likewise,  | is just a plain old  '|' character and not an alternation operator. So the character class finally becomes something like  [\banthe|] depending on just what  \b becomes.