while (<MYFILE>) {

    if (/^FH$/ .. /^FT$/) {

        # $_ is a line within FH and FT lines
        # including the delimiting lines

    }

    if (/^BH$/ ... /^BH$|^FT$/) {

        # $_ is a line between BH lines, or between
        # a BH and FT line

    }