Hi isha, I am not clear with your question. The problem may be in variable name $t1, which should be $t and inside character class range should be [A-Z], since you are using underscore seprately in the character class.

if($t1 !~ /A_[A_Z0-9_]+/i)

it should be

if($t !~ /A_[A-Z0-9_]+/i)

Output:
-------
$CD = 'A_TAG_HSSL_LBA_1';  #matches
$CD = 'A_TAG_HSSL_LBAB_1'; #not matches
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