for ( 'A_TAG_HSSL_LBA_1', 'A_TAG_HSSL_LBAB_1' ) {
    if( ! /A_[A_Z0-9_]+/i) {
        print "$_ is not matching\n";
    }
    else {
        print "$_ matches\n";
    }
}

##</code><code>##

A_TAG_HSSL_LBA_1 matches
A_TAG_HSSL_LBAB_1 is not matching

##</code><code>##

/^           # start of string
  A_         # must start "A_"
  [A-Z0-9_]+ # at least one capital, digit or underscore
 $           # end of string
/ix          # allow spaces and comments inside regexp
             # and ignore case

##</code><code>##