I think your problem in the first regex is the braces ("square brackets") in place of parentheses. When I tried a similar regex, I got the expected results:

$ perl -Mstrict -Mwarnings -le '
my $str = q{This is a test of a regex to remove the words "a", "an", a
+nd "the" from a sentence.}; 
print q{Before: }, $c; 
$c =~ s/\b(an?|the)\b/gimsx; 
print q{After:  };
'
This is a test of a regex to remove the words "a", "an", and "the" fro
+m a sentence.
This is  test of  regex to remove  words "", "", and "" from  sentence
+.
[download]

Hope that helps.