Display specific headers

you don't need matchclass as it is for datagroup matching.

This is a bit of a basic programming errror. All you need to do is just skip the loop iteration you don't want using continue operator:

foreach header [HTTP::header names] {
        if {$header eq "BASE64"} {continue}
        set HEADER_RESPONSE "$HEADER_RESPONSE > $header = [HTTP::header $header]
"
 } 

Hope this helps,

Sam