c:\@Work\Perl>perl -wMstrict -le
"my $rxExcl = do {
     my @excl = qw{ if else 1'b0 end };
     local $\" = q{^|};
     qr{(?x) (?: @excl ) \b};
     };
 print $rxExcl;
"
(?^:(?x) (?: if|else|1'b0|end ) \b)