As others have said, you can't write ifs like that in Perl 5. Depending on exactly what it is you want, here's another way

for ( '"KOTB"', '"KBBA"', '"KBBC"', '"KBBM"', '"KBBT"' ) {
    if ( $fields[6] eq $_ ) {
        # do something
    }
}
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@fields[6] is better written $fields[6] (warnings should have told you about that). @fields[6] an array slice that's described in perldata.