I agree with matching against a particular field rather than against the entire string, and with using a character class rather than several regexes | matches in tandem.

I have some comments regarding implementation details. I'm forced to admit, however, that because I don't really know DAN0207's requirements, these comments may be meaningless. That said, I forge ahead.

Firstly, the  /SMS[1HI]/i match against the extracted $SMSfield field allows a field like 'xSMSIx' to be accepted. This match could benefit from anchor assertions:  / \A SMS [1HI] \z /xi rejects this field.

Secondly, I find the use of the global  /i flag problematic. In the OPed code statement
    $$blk_ref = 'SMSblk' if $$blk_ref =~ /SMSi/i || ... || $$blk_ref =~ /SMS1/;
the  /i modifier is only present in matches with an  i I h H suffix, not with the numeric suffix. This suggests (and again, I'm only guessing) that the 'SMS' subfield of the field in question should not be matched case-insensitively. If that's so, a match of
    / \A SMS [1hHiI] \z /x
(which I personally prefer) or
    / \A SMS (?i) [1HI] \z /x
will reject the 'SmsI' field and all like it.