c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $text = q{x aren't y ain't z isn't w won't u 't v daren't t};
 print qq{'$text'};
 ;;
 my $sq_t = qr{ 't }xms;
 my $skip_sq_t = qr{ aren | isn }xms;
 ;;
 my $exceptions_sq_t = qr{ \b $skip_sq_t $sq_t (*SKIP) (*FAIL) }xms;
 ;;
 $text =~ s{ $exceptions_sq_t? (?<= [[:alpha:]]) (?= $sq_t) }{ }xmsg;
 print qq{'$text'};
"
'x aren't y ain't z isn't w won't u 't v daren't t'
'x aren't y ain 't z isn't w won 't u 't v daren 't t'