while ( <RULE> ) {
  if (/^STMT/.../^STMT/) {
    print unless /^STMT/;
  }
}

##</code><code>##

/^STMT/.../^STMT/ and !/^STMT/ and print while <RULE>;
__END__
A
B
C
Y
Z

##</code><code>##

  if (/^STMT\d+=BEGIN/../^STMT\d+=END/)