\b(?:a(?:bout|bove|cross|fter|fterwards|))\b

##</code><code>##

use constant PREFIXSIZE => 1;

my @stopwords = qw(
   a about above across after afterwards );
   # presumably much larger, with other first letters...

my $curfirst;
my %buildhash = ();

# create first => rest mapping:
for ( @stopwords )
{
   my $first = substr( $_, PREFIXSIZE, 1, "" );
   push @{$buildhash{$first}}, $_;
}

# use letter hash to build regex
my $regex = '';
for ( sort keys %buildhash )
{
   $regex .= "(?:\\b$_";
   if ( @{$buildhash{$_}} > 1 )
   {
      $regex .= "(?:" . 
                join( '|',  @{$buildhash{$_}} ) . ")\\b)|";
   } else {
      $regex .= ${$buildhash{$_}}[0]  . "\\b)|";
   }
}

# ditch trailing pipe
substr( $regex, -1, 1, '' );
print $regex, "\n";

__END__


prints:
\b(?:a(?:|bout|bove|cross|fter|fterwards))\b